Driving method of semiconductor device and electronic device

ABSTRACT

A driving method of a semiconductor device that takes three-dimensional images with short duration is provided. In a first step, a light source starts to emit light, and first potential corresponding to the total amount of light received by a first photoelectric conversion element and a second photoelectric conversion element is written to a first charge accumulation region. In a second step, the light source stops emitting light and second potential corresponding to the total amount of light received by the first photoelectric conversion element and the second photoelectric conversion element is written to a second charge accumulation region. In a third step, first data corresponding to the potential written to the first charge accumulation region is read. In a fourth step, second data corresponding to the potential written to the second charge accumulation region is read.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/693,483, filed Nov. 25, 2019, now allowed, which is a continuation ofU.S. application Ser. No. 15/642,390, filed Jul. 6, 2017, now U.S. Pat.No. 10,516,842, which is a continuation of U.S. application Ser. No.14/790,801, filed Jul. 2, 2015, now U.S. Pat. No. 9,729,809, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2014-143256 on Jul. 11, 2014, all of which are incorporatedby reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice and a driving method thereof. Specifically, the present inventionrelates to a solid-state imaging device including a plurality of pixelsprovided with photosensors and to a method for driving the solid-stateimaging device. Further, the present invention relates to an electronicdevice including the solid-state imaging device.

Note that one embodiment of the present invention is not limited to theabove technical field. For example, one embodiment of the presentinvention relates to an object, a method, or a manufacturing method. Oneembodiment of the present invention relates to a process, a machine,manufacture, or a composition of matter. Further, one embodiment of thepresent invention relates to a memory device, a processor, a drivingmethod thereof, or a manufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. Thus, a semiconductor element such as a transistor or adiode and a semiconductor circuit are semiconductor devices. A displaydevice, a light-emitting device, a lighting device, an electro-opticaldevice, a solid-state imaging device, an electronic device, and the likemay include a semiconductor element or a semiconductor circuit.Therefore, a display device, a light-emitting device, a lighting device,an electro-optical device, a solid-state imaging device, an electronicdevice, and the like include a semiconductor device in some cases.

2. Description of the Related Art

A photosensor using an amplification function of a metal oxidesemiconductor (MOS) transistor, called a complementary metal oxidesemiconductor (CMOS) sensor, can be fabricated through a general CMOSprocess. Thus, manufacturing cost of an imaging device including a CMOSsensor in each pixel can be low, and a semiconductor device having aphotosensor and a display element formed over one substrate can berealized. Further, the drive voltage of a CMOS sensor is lower than thatof a charge coupled device (CCD) sensor, so that power consumption ofthe solid-state imaging device can be kept low.

A solid-state imaging device including a CMOS sensor generally employs,for imaging, a rolling shutter method in which an operation toaccumulate charge in a photodiode and an operation to read the chargeare sequentially performed row by row (see Patent Document 1). In somecases, such a solid-state imaging device employs a global shutter methodin which all the pixels are subjected to an operation to accumulatecharge at a time, instead of the rolling shutter method.

Non-Patent Document 1 discloses an example in which three-dimensionalimaging of an object is performed by detecting light reflected from theobject.

As Non-Patent Document 1 describes, the channel width of a transistor,which transfers charge obtained through photoelectric conversion of aphotodiode to a charge accumulation region, is increased to improve thetransfer efficiency.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2009-141717-   Non-Patent Document 1: S. J. Kim et al., “A Three-Dimensional    Time-of-Flight CMOS Image Sensor With Pinned-Photodiode Pixel    Structure”, IEEE Electron Device Letters, November 2010, Vol. 31,    No. 11, pp. 1272-1274.

SUMMARY OF THE INVENTION

However, increase in channel width of a transistor may increaseoff-state current, leading to deterioration in characteristics ofholding accumulated charge. An object of one embodiment of the presentinvention is to provide a solid-state imaging device capable of takinghigh-quality images, or the like. Another object of one embodiment ofthe present invention is to provide a solid-state imaging device with ashort duration of imaging, or the like. Another object of one embodimentof the present invention is to provide a low-power-consumptionsolid-state imaging device or the like. Another object of one embodimentof the present invention is to provide a driving method thereof.

Another object of one embodiment of the present invention is to providea solid-state imaging device capable of three-dimensional imaging with ashort duration of imaging. Another object of one embodiment of thepresent invention is to provide a solid-state imaging device capable ofhigh-definition two-dimensional imaging. Another object of oneembodiment of the present invention is to provide an imaging device withhigh reliability. Another object of one embodiment of the presentinvention is to provide a driving method thereof.

Note that the descriptions of these objects do not preclude theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention is a driving method of asemiconductor device that includes a first circuit, a second circuit, athird transistor, and a light source. The first circuit includes a firstphotoelectric conversion element, a first transistor, and a first chargeaccumulation region. The second circuit includes a second photoelectricconversion element, a second transistor, and a second chargeaccumulation region. One of a source and a drain of the first transistoris electrically connected to the first photoelectric conversion element.The other of the source and the drain of the first transistor iselectrically connected to the first charge accumulation region. One of asource and a drain of the second transistor is electrically connected tothe second photoelectric conversion element. The other of the source andthe drain of the second transistor is electrically connected to thesecond charge accumulation region. One of a source and a drain of thethird transistor is electrically connected to the first photoelectricconversion element. The other of the source and the drain of the thirdtransistor is electrically connected to the second photoelectricconversion element. The driving method of the semiconductor device ofone embodiment of the present invention includes a first step, a secondstep, a third step, and a fourth step. In the first step, the lightsource starts to emit light. In the first step, the first transistor andthe third transistor are on. In the first step, the second transistor isoff. In the first step, first potential corresponding to a total amountof light received by the first photoelectric conversion element and thesecond photoelectric conversion element is written to the first chargeaccumulation region. In the second step, the light source stops emittinglight. In the second step, the first transistor is off. In the secondstep, the second transistor and the third transistor are on. In thesecond step, second potential corresponding to a total amount of lightreceived by the first photoelectric conversion element and the secondphotoelectric conversion element is written to the second chargeaccumulation region. In the third step, the first transistor and thesecond transistor are off. In the third step, first data correspondingto the first potential written to the first charge accumulation regionis read. In the fourth step, the first transistor and the secondtransistor are off. In the fourth step, second data corresponding to thesecond potential written to the second charge accumulation region isread.

In the above structure, it is preferable that: the semiconductor deviceinclude an imaging device, the imaging device image an object in thefirst step and the second step, the object be irradiated with lightemitted from the light source in the first step, and a distance betweenthe imaging device and the object be calculated based on the first dataand the second data. In the above structure, it is preferable that thefirst transistor and the second transistor each include an oxidesemiconductor. In the above structure, it is preferable that the firstphotoelectric conversion element and the second photoelectric conversionelement each include a PIN junction.

In the above structure, it is preferable that: the first circuit includea fourth transistor, the second circuit include a fifth transistor, agate of the fourth transistor be electrically connected to the firstcharge accumulation region, a gate of the fifth transistor beelectrically connected to the second charge accumulation region, the oneof the source and the drain of the first transistor be electricallyconnected to the one of the source and the drain of the thirdtransistor, and the one of the source and the drain of the secondtransistor be electrically connected to the other of the source and thedrain of the third transistor.

According to one embodiment of the present invention, a solid-stateimaging device capable of taking high-quality images, a solid-stateimaging device with a short duration of imaging, or the like can beprovided. According to one embodiment of the present invention, alow-power-consumption solid-state imaging device or the like can beprovided. According to one embodiment of the present invention, adriving method thereof can be provided.

According to one embodiment of the present invention, a solid-stateimaging device capable of three-dimensional imaging with a shortduration of imaging, a solid-state imaging device capable ofhigh-definition two-dimensional imaging, or an imaging device with highreliability can be provided. According to one embodiment of the presentinvention, a driving method thereof can be provided.

Note that one embodiment of the present invention is not limited tothese effects. For example, depending on circumstances or conditions,one embodiment of the present invention might produce another effect.Furthermore, depending on circumstances or conditions, one embodiment ofthe present invention might not produce any of the above effects.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B each illustrate a configuration example of an imagingdevice of one embodiment of the present invention;

FIGS. 2A and 2B each illustrate a configuration example of a pixel;

FIGS. 3A to 3D each illustrate a configuration example of a pixel;

FIGS. 4A and 4B each illustrate a structure example of a pixel;

FIG. 5 illustrates a circuit configuration example of a pixel;

FIG. 6 is a timing chart showing an example of an imaging operation;

FIGS. 7A and 7B are each a circuit diagram showing an example of animaging operation;

FIGS. 8A and 8B are each a circuit diagram showing an example of animaging operation;

FIGS. 9A and 9B are each a circuit diagram showing an example of animaging operation;

FIGS. 10A and 10B illustrate a structure example of a pixel;

FIG. 11 illustrates a configuration example of a peripheral circuit;

FIG. 12 is a timing chart showing an example of an imaging operation;

FIGS. 13A and 13B are each a circuit diagram showing an example of animaging operation;

FIGS. 14A and 14B are each a circuit diagram showing an example of animaging operation;

FIGS. 15A and 15B are each a circuit diagram showing an example of animaging operation;

FIGS. 16A and 16B are each a circuit diagram showing an example of animaging operation;

FIG. 17 illustrates a configuration example of a pixel;

FIG. 18 is a timing chart showing an example of an imaging operation;

FIG. 19 is a timing chart showing an example of an imaging operation;

FIGS. 20A and 20B each illustrate a circuit configuration example of apixel;

FIG. 21 illustrates a structure example of an imaging device;

FIG. 22 illustrates a structure example of a transistor;

FIG. 23 shows an energy band structure;

FIGS. 24A and 24B each illustrate an example of a transistor;

FIGS. 25A to 25E each illustrate an example of a circuit configuration;

FIGS. 26A to 26C each illustrate an example of an optical sensor;

FIGS. 27A and 27B each illustrate an example of a circuit configuration;

FIGS. 28A to 28C each illustrate an example of a circuit configuration;

FIGS. 29A1, 29A2, 29B1, and 29B2 each illustrate one embodiment of atransistor;

FIGS. 30A1 to 30A3 and 30B1 and 30B2 each illustrate one embodiment of atransistor;

FIGS. 31A to 31C illustrate one embodiment of a transistor;

FIGS. 32A to 32C illustrate one embodiment of a transistor;

FIGS. 33A to 33F are diagrams illustrating electronic devices accordingto one embodiment of the present invention;

FIG. 34 illustrates a structure example of an imaging device;

FIGS. 35A1 to 35A3 and 35B1 to 35B3 illustrate structure examples of animaging device;

FIG. 36 illustrates an example of an imaging operation;

FIG. 37 illustrates a structure example of an imaging device; and

FIG. 38 illustrates a structure example of an imaging device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the drawings. However, the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that modes and details disclosed herein can bemodified in various ways. Further, the present invention is notconstrued as being limited to description of the embodiments. Note thatin all drawings used to illustrate the embodiments, portions that areidentical or portion having similar functions are denoted by the samereference numerals, and their repetitive description may be omitted.

In addition, in this specification and the like, the term such as an“electrode” or a “wiring” does not limit a function of a component. Forexample, an “electrode” is used as part of a “wiring” in some cases, andvice versa. Further, the term “electrode” or “wiring” can also mean acombination of a plurality of “electrodes” and “wirings” formed in anintegrated manner.

For example, in this specification and the like, when it is explicitlydescribed that X and Y are connected, the case where X and Y areelectrically connected, the case where X and Y are functionallyconnected, and the case where X and Y are directly connected areincluded therein. Accordingly, another element may be provided betweenelements having a connection relation illustrated in drawings and texts,without limitation on a predetermined connection relation, for example,the connection relation illustrated in the drawings and the texts.

Here, X and Y each denote an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, a layer,or the like).

Examples of the case where X and Y are directly connected include thecase where an element that allows an electrical connection between X andY (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, and a load) is notconnected between X and Y, and the case where X and Y are connectedwithout the element that allows the electrical connection between X andY provided therebetween.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. A switch is controlled to be on or off Thatis, a switch has a function of becoming conducting or not conducting(being turned on or off) to determine whether current flows therethroughor not. Alternatively, the switch has a function of selecting andchanging a current path. Note that the case where X and Y areelectrically connected includes the case where X and Y are directlyconnected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a step-upcircuit or a step-down circuit) or a level shifter circuit for changingthe potential level of a signal; a voltage source; a current source; aswitching circuit; an amplifier circuit such as a circuit that canincrease signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; amemory circuit; and/or a control circuit) can be connected between X andY. Note that for example, in the case where a signal output from X istransmitted to Y, even when another circuit is provided between X and Y,X and Y are functionally connected. Note that the case where X and Y arefunctionally connected includes the case where X and Y are directlyconnected and the case where X and Y are electrically connected.

Note that in this specification and the like, an explicit description “Xand Y are electrically connected” means that X and Y are electricallyconnected (i.e., the case where X and Y are connected with anotherelement or another circuit provided therebetween), X and Y arefunctionally connected (i.e., the case where X and Y are functionallyconnected with another circuit provided therebetween), and X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween). That is, inthis specification and the like, the explicit description “X and Y areelectrically connected” is the same as the description “X and Y areconnected”.

Note that, for example, the case where a source (or a first terminal orthe like) of a transistor is electrically connected to X through (or notthrough) Z1 and a drain (or a second terminal or the like) of thetransistor is electrically connected to Y through (or not through) Z2,or the case where a source (or a first terminal or the like) of atransistor is directly connected to one part of Z1 and another part ofZ1 is directly connected to X while a drain (or a second terminal or thelike) of the transistor is directly connected to one part of Z2 andanother part of Z2 is directly connected to Y, can be expressed by usingany of the following expressions.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit configuration is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Other examples of the expressions include, “a source (or a firstterminal or the like) of a transistor is electrically connected to Xthrough at least a first connection path, the first connection path doesnot include a second connection path, the second connection path is apath between the source (or the first terminal or the like) of thetransistor and a drain (or a second terminal or the like) of thetransistor, Z1 is on the first connection path, the drain (or the secondterminal or the like) of the transistor is electrically connected to Ythrough at least a third connection path, the third connection path doesnot include the second connection path, and Z2 is on the thirdconnection path”. It is also possible to use the expression “a source(or a first terminal or the like) of a transistor is electricallyconnected to X through at least Z1 on a first connection path, the firstconnection path does not include a second connection path, the secondconnection path includes a connection path through the transistor, adrain (or a second terminal or the like) of the transistor iselectrically connected to Y through at least Z2 on a third connectionpath, and the third connection path does not include the secondconnection path”. Still another example of the expression is “a source(or a first terminal or the like) of a transistor is electricallyconnected to X through at least Z1 on a first electrical path, the firstelectrical path does not include a second electrical path, the secondelectrical path is an electrical path from the source (or the firstterminal or the like) of the transistor to a drain (or a second terminalor the like) of the transistor, the drain (or the second terminal or thelike) of the transistor is electrically connected to Y through at leastZ2 on a third electrical path, the third electrical path does notinclude a fourth electrical path, and the fourth electrical path is anelectrical path from the drain (or the second terminal or the like) ofthe transistor to the source (or the first terminal or the like) of thetransistor”. When the connection path in a circuit configuration isdefined by an expression similar to the above examples, a source (or afirst terminal or the like) and a drain (or a second terminal or thelike) of a transistor can be distinguished from each other to specifythe technical scope.

Note that these expressions are examples and there is no limitation onthe expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, and a layer).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

Note that in this specification and the like, a transistor can be formedusing a variety of substrates. The type of a substrate is not limited toa certain type. As the substrate, a semiconductor substrate (e.g., asingle crystal substrate or a silicon substrate), an SOI substrate, aglass substrate, a quartz substrate, a plastic substrate, a metalsubstrate, a stainless steel substrate, a substrate including stainlesssteel foil, a tungsten substrate, a substrate including tungsten foil, aflexible substrate, an attachment film, paper including a fibrousmaterial, a base material film, or the like can be used, for example. Asan example of a glass substrate, a barium borosilicate glass substrate,an aluminoborosilicate glass substrate, soda lime glass substrate, andthe like can be given. For a flexible substrate, a flexible syntheticresin such as plastics typified by polyethylene terephthalate (PET),polyethylene naphthalate (PEN), and polyether sulfone (PES), or acryliccan be used, for example. For an attachment film, polypropylene,polyester, polyvinyl fluoride, polytetrafluoroethylene (PTFE), orpolyvinyl chloride can be used, for example. For a base material film,polyester, polyamide, polyimide, an inorganic vapor deposition film,paper, or the like can be used, for example. Specifically, when atransistor is formed using a semiconductor substrate, a single crystalsubstrate, an SOI substrate, or the like, it is possible to form atransistor with few variations in characteristics, size, shape, or thelike and with high current supply capability and a small size. Byforming a circuit with the use of such a transistor, power consumptionof the circuit can be reduced or the circuit can be highly integrated.

Note that a transistor may be formed using one substrate, and then thetransistor may be transferred to another substrate. In addition to theabove substrates over which the transistor can be formed, a papersubstrate, a cellophane substrate, a stone substrate, a wood substrate,a cloth substrate (including a natural fiber (e.g., silk, cotton, orhemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), aregenerated fiber (e.g., acetate, cupra, rayon, or regeneratedpolyester), or the like), a leather substrate, a rubber substrate, orthe like can be used as a substrate to which the transistor istransferred. By using such a substrate, a transistor with excellentproperties or a transistor with low power consumption can be formed, adevice with high durability can be formed, heat resistance can beprovided, or reduction in weight or thickness can be achieved.

The position, size, range, and the like of each component illustrated inthe drawings and the like are not accurately represented in some casesto facilitate understanding of the invention. Therefore, the disclosedinvention is not necessarily limited to the position, the size, therange, or the like disclosed in the drawings and the like. For example,in the actual manufacturing process, a resist mask or the like might beunintentionally reduced in size by treatment such as etching, which isnot illustrated in some cases for easy understanding.

Especially in a top view (also referred to as a plan view), somecomponents might not be illustrated for easy understanding. In addition,some hidden lines and the like might not be shown.

Note that the term “over” or “under” in this specification and the likedoes not necessarily mean that a component is placed “directly on” or“directly below” and “directly in contact with” another component. Forexample, the expression “electrode B over insulating layer A” does notnecessarily mean that the electrode B is on and in direct contact withthe insulating layer A and can mean the case where another component isprovided between the insulating layer A and the electrode B.

Further, functions of the source and the drain might be switcheddepending on operation conditions, e.g., when a transistor having adifferent polarity is employed or a direction of current flow is changedin circuit operation. Accordingly, it is difficult to define whicheverelectrode serves as a source or a drain. Thus, the terms “source” and“drain” can be switched in this specification.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “perpendicular” or “orthogonal” indicates that theangle formed between two straight lines is greater than or equal to 80°and less than or equal to 100°, and accordingly also includes the casewhere the angle is greater than or equal to 85° and less than or equalto 95°.

A voltage usually refers to a potential difference between a givenpotential and a reference potential (e.g., a source potential or aground potential (a GND potential)). A voltage can be referred to as apotential and vice versa.

Note that a “semiconductor” includes characteristics of an “insulator”in some cases when the conductivity is sufficiently low, for example.Thus, a “semiconductor” and an “insulator” can be replaced with eachother. Furthermore, a “semiconductor” and an “insulator” cannot bestrictly distinguished from each other because a border between the“semiconductor” and the “insulator” is not clear. Accordingly, a“semiconductor” in this specification can be replaced with an“insulator” in some cases.

Furthermore, a “semiconductor” includes characteristics of a “conductor”in some cases when the conductivity is sufficiently high, for example.Thus, a “semiconductor” and a “conductor” can be replaced with eachother. Furthermore, a “semiconductor” and a “conductor” cannot bestrictly distinguished from each other because a border between the“semiconductor” and the “conductor” is not clear. Accordingly, a“semiconductor” in this specification can be replaced with a “conductor”in some cases.

Note that an impurity in a semiconductor refers to, for example,elements other than the main components of the semiconductor. Forexample, an element with a concentration lower than 0.1 atomic % can beregarded as an impurity. When an impurity is contained, the density ofstates (DOS) in a semiconductor may be increased, the carrier mobilitymay be decreased, or the crystallinity may be decreased. In the casewhere the semiconductor is an oxide semiconductor, examples of animpurity that changes the characteristics of the semiconductor includeGroup 1 elements, Group 2 elements, Group 14 elements, Group 15elements, and transition metals other than the main components; specificexamples are hydrogen (included in water), lithium, sodium, silicon,boron, phosphorus, carbon, and nitrogen. In the case of an oxidesemiconductor, oxygen vacancy may be formed by entry of impurities suchas hydrogen. Further, in the case where the semiconductor is silicon,examples of an impurity that changes characteristics of thesemiconductor include oxygen, Group 1 elements except hydrogen, Group 2elements, Group 13 elements, and Group 15 elements.

Note that ordinal numbers such as “first” and “second” in thisspecification and the like are used in order to avoid confusion amongcomponents and do not denote the priority or the order such as the orderof steps or the stacking order. A term without an ordinal number in thisspecification and the like might be provided with an ordinal number in aclaim in order to avoid confusion among components. In addition, a termwith an ordinal number in this specification and the like might beprovided with a different ordinal number in a claim. Moreover, a termwith an ordinal number in this specification and the like might not beprovided with any ordinal number in a claim.

Note that in this specification, the channel length refers to, forexample, a distance between a source (a source region or a sourceelectrode) and a drain (a drain region or a drain electrode) in a regionwhere a semiconductor (or a portion where a current flows in asemiconductor when a transistor is on) and a gate electrode overlap witheach other or a region where a channel is formed in a top view of thetransistor. In one transistor, channel lengths in all regions are notnecessarily the same. In other words, the channel length of onetransistor is not limited to one value in some cases. Therefore, in thisspecification, the channel length is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

The channel width refers to, for example, the length of a portion wherea source and a drain face each other in a region where a semiconductor(or a portion where a current flows in a semiconductor when a transistoris on) and a gate electrode overlap with each other, or a region where achannel is formed. In one transistor, channel widths in all regions donot necessarily have the same value. In other words, the channel widthof one transistor is not limited to one value in some cases. Therefore,in this specification, a channel width is any one of values, the maximumvalue, the minimum value, or the average value in a region where achannel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is actually formed (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of a transistor (hereinafter referred to as an apparent channelwidth) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is higher than the proportion of a channel region formedin a top surface of a semiconductor in some cases. In that case, aneffective channel width obtained when a channel is actually formed isgreater than an apparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example, toestimate an effective channel width from a design value, it is necessaryto assume that the shape of a semiconductor is known as an assumptioncondition. Therefore, in the case where the shape of a semiconductor isnot known accurately, it is difficult to measure an effective channelwidth accurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Further, in this specification, in the casewhere the term “channel width” is simply used, it may denote asurrounded channel width or an apparent channel width. Alternatively, inthis specification, in the case where the term “channel width” is simplyused, it may denote an effective channel width in some cases. Note thatthe values of a channel length, a channel width, an effective channelwidth, an apparent channel width, a surrounded channel width, and thelike can be determined by obtaining and analyzing a cross-sectional TEMimage and the like.

Note that in the case where electric field mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from one in the case where an effective channelwidth is used for the calculation is obtained in some cases.

In this specification and the like, the high power supply potentialV_(DD) (hereinafter also simply referred to as V_(DD) or H potential) isa power supply potential higher than the low power supply potentialV_(SS). The low power supply potential V_(SS) (hereinafter also simplyreferred to as V_(SS) or L potential) is a power supply potential lowerthan the high power supply potential V_(DD). In addition, a groundpotential can be used as V_(DD) or V_(SS). For example, in the casewhere a ground potential is used as V_(DD), V_(SS) is lower than theground potential, and in the case where a ground potential is used asV_(SS), V_(DD) is higher than the ground potential.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Embodiment 1

In this embodiment, an imaging device of one embodiment of the presentinvention will be described with reference to the drawings.

[Configuration Example of Imaging Device 100]

FIG. 1A is a plan view illustrating a configuration example of animaging device 100 of one embodiment of the present invention. Theimaging device 100 includes a pixel portion 110, and peripheral circuits(a first peripheral circuit 260, a second peripheral circuit 270, athird peripheral circuit 280, and a fourth peripheral circuit 290) fordriving the pixel portion 110. The pixel portion 110 includes aplurality of pixels 111 arranged in a matrix with p rows and q columns(p and q are each a natural number greater than or equal to 2). Thefirst to fourth peripheral circuits (260 to 290) are connected to theplurality of pixels 111 and each have a function of supplying a signalfor driving the plurality of pixels 111. In this specification and thelike, the first to fourth peripheral circuits (260 to 290) and the likeare referred to as “peripheral circuit” or “driving circuit” in somecases. For example, the first peripheral circuit 260 can be regarded aspart of the peripheral circuit.

The imaging device 100 preferably includes a light source 190. The lightsource 190 can emit light P1.

The peripheral circuit includes at least one of a logic circuit, aswitch, a buffer, an amplifier circuit, and a converter circuit. Theperipheral circuit may be formed over a substrate over which the pixelportion 110 is formed. Alternatively, a part or whole of the peripheralcircuit may be fabricated using a semiconductor device such as an IC.Note that in the peripheral circuit, at least one of the first to fourthperipheral circuits (260 to 290) may be omitted. For example, when oneof the first peripheral circuit 260 and the fourth peripheral circuit290 additionally has a function of the other of the first peripheralcircuit 260 and the fourth peripheral circuit 290, the other of thefirst peripheral circuit 260 and the fourth peripheral circuit 290 maybe omitted. For another example, when one of the second peripheralcircuit 270 and the third peripheral circuit 280 additionally has afunction of the other of the second peripheral circuit 270 and the thirdperipheral circuit 280, the other of the second peripheral circuit 270and the third peripheral circuit 280 may be omitted. For anotherexample, when one of the first to fourth peripheral circuits (260 to290) additionally has functions of the other circuits, the othercircuits may be omitted.

As illustrated in FIG. 1B, the pixels 111 may be provided to be inclinedin the pixel portion 110 included in the imaging device 100. When thepixels 111 are inclined, the space between the pixels in the rowdirection and the column direction (pitch) can be decreased.Accordingly, the quality of an image taken with the imaging device 100can be improved.

For example, the first peripheral circuit 260 or the fourth peripheralcircuit 290 has a function of processing analog signals output from thepixels 111. For example, the first peripheral circuit 260 may include asignal processing circuit 271, a column driver circuit 272, an outputcircuit 273, and the like, as shown in FIG. 11 .

The signal processing circuit 271 shown in FIG. 11 includes circuits 274each of which is provided for each column. The circuit 274 can have afunction of performing signal processing such as removal of noise andanalog-digital conversion. The circuit 274 shown in FIG. 11 has afunction of analog-digital conversion. The signal processing circuit 271can function as a column-parallel (column type) analog-digitalconversion device.

The circuit 274 includes a comparator 274 a and a counter circuit 274 b.The comparator 274 a has a function of comparing potentials of an analogsignal input from a wiring 999 that is provided for each column and areference potential signal (e.g., a ramp signal) input from a wiring277. A clock signal is input to the counter circuit 274 b from a wiring278. The counter circuit 274 b has a function of measuring the length ofa period in which a first value is output owing to the comparisonoperation in the comparator 274 a and holding the measurement result asan N-bit digital value.

The column driver circuit 272 is also referred to as a column selectioncircuit, a horizontal driver circuit, or the like. The column drivercircuit 272 generates a selection signal for selecting a column fromwhich a signal is read. The column driver circuit 272 can be formedusing a shift register or the like. Columns are sequentially selected bythe column driver circuit 272, and a signal output from the circuit 274in the selected column is input to the output circuit 273 via a wiring279. The wiring 279 can function as a horizontal transfer line.

A signal input to the output circuit 273 is processed in the outputcircuit 273, and is output outside the imaging device 100. The outputcircuit 273 can be formed using a buffer circuit, for example. Theoutput circuit 273 may have a function of controlling the timing atwhich a signal is output outside the imaging device 100.

The second peripheral circuit 270 or the third peripheral circuit 280has a function of generating and outputting a selection signal forselecting a pixel 111 from which a signal is read. Note that the secondperipheral circuit 270 or the third peripheral circuit 280 may also bereferred to as a row selection circuit or a vertical driver circuit.

As shown in FIGS. 35A1 and 35B1, the imaging device 100 may be bent.FIG. 35A1 illustrates a state in which the imaging device 100 is bent inthe direction of two-dot chain line X1-X2. FIG. 35A2 is across-sectional view illustrating a portion indicated by two-dot chainline X1-X2 in FIG. 35A1. FIG. 35A3 is a cross-sectional viewillustrating a portion indicated by two-dot chain line Y1-Y2 in FIG.35A1.

FIG. 35B1 illustrates a state where the imaging device 100 is bent inthe direction of two-dot chain line X3-X4 and the direction of two-dotchain line Y3-Y4. FIG. 35B2 is a cross-sectional view illustrating aportion indicated by two-dot chain line X3-X4 in FIG. 35B1. FIG. 35B3 isa cross-sectional view illustrating a portion indicated by two-dot chainline Y3-Y4 in FIG. 35B1.

The bent imaging device 100 enables the curved field and astigmatism tobe reduced. Thus, the optical design of a lens and the like, which isused in combination of the imaging device 100, can be facilitated. Forexample, the number of lenses used for aberration correction can bereduced; accordingly, a reduction of size or weight of an imaging deviceor semiconductor device using the imaging device 100 can be facilitated.In addition, the quality of a taken image can be improved.

[Structure Example of Pixel 111]

The pixel 111 included in the imaging device 100 is formed with aplurality of subpixels 112, and each subpixel 112 is combined with afilter which transmits light with a specific wavelength band (colorfilter), whereby data for achieving color image display can be obtained.

FIG. 2A is a plan view showing an example of the pixel 111 with which acolor image is obtained. The pixel 111 illustrated in FIG. 2A includes asubpixel 112 provided with a color filter transmitting light with a red(R) wavelength band (hereinafter also referred to as “subpixel 112R”), asubpixel 112 provided with a color filter transmitting light with agreen (G) wavelength band (hereinafter also referred to as “subpixel112G”), and a subpixel 112 provided with a color filter transmittinglight with a blue (B) wavelength band (hereinafter also referred to as“subpixel 112B”). The subpixel 112 can function as a photosensor.

The subpixels 112 (the subpixel 112R, the subpixel 112G, and thesubpixel 112B) are electrically connected to a wiring 131, a wiring 141,a wiring 144, a wiring 146, and a wiring 135. In addition, the subpixel112R, the subpixel 112G, and the subpixel 112B are connected torespective wirings 137 which are independently provided. In thisspecification and the like, for example, the wiring 144 and the wiring146 that are connected to the pixel 111 in the n-th row are referred toas a wiring 144[n] and a wiring 146[n]. For example, the wiring 137connected to the pixel 111 in the m-th column is referred to as a wiring137[m]. Note that in FIG. 2A, the wirings 137 connected to the subpixel112R, the subpixel 112G, and the subpixel 112B in the pixel 111 in them-th column are referred to as a wiring 137 [m] R, a wiring 137[m]G, anda wiring 137[m]B, respectively. The subpixels 112 are electricallyconnected to the peripheral circuit through the above wirings.

The imaging device 100 in this embodiment has a structure in which thesubpixels 112, which are provided with color filters transmitting lightwith the same wavelength band and are arranged in the pixels 111adjacent to each other, are connected to each other via a switch. FIG.2B shows a connection example of the subpixels 112: the subpixel 112 inthe pixel 111 arranged in the n-th (n is a natural number greater thanor equal to 1 and less than or equal top) row and the m-th (m is anatural number greater than or equal to 1 and less than or equal to q)column and the subpixel 112 in the adjacent pixel 111 arranged in the(n+1)-th row and the m-th column. In FIG. 2B, the subpixel 112R arrangedin the n-th row and the m-th column and the subpixel 112R arranged inthe (n+1)-th row and the m-th column are connected to each other via aswitch 201. The subpixel 112G arranged in the n-th row and the m-thcolumn and the subpixel 112G arranged in the (n+1)-th row and the m-thcolumn are connected to each other via a switch 202. The subpixel 112Barranged in the n-th row and the m-th column and the subpixel 112Barranged in the (n+1)-th row and the m-th column are connected to eachother via a switch 203.

The color filter used in the subpixel 112 is not limited to red (R),green (G), and blue (B) color filters, and as illustrated in FIG. 3A,color filters that transmit light of cyan (C), yellow (Y), and magenta(M) may be used. The subpixels 112 that sense light with three differentwavelength bands are provided in one pixel 111, whereby a full-colorimage can be obtained.

FIG. 3B shows the pixel 111 including the subpixel 112 provided with acolor filter transmitting yellow (Y) light, in addition to the subpixels112 provided with the color filters transmitting red (R), green (G), andblue (B) light. FIG. 3C illustrates the pixel 111 including the subpixel112 provided with a color filter transmitting blue (B) light, inaddition to the subpixels 112 provided with the color filterstransmitting cyan (C), yellow (Y), and magenta (M) light. When thesubpixels 112 sensing light with four different wavelength bands areprovided in one pixel 111, the reproducibility of colors of an obtainedimage can be increased.

For example, in FIG. 2A, in regard to the subpixel 112 sensing a redwavelength band, the subpixel 112 sensing a green wavelength band, andthe subpixel 112 sensing a blue wavelength band, the pixel number ratio(or the ratio of light receiving area) thereof is not necessarily 1:1:1.The pixel number ratio (the ratio of light receiving area) of red andgreen to blue may be 1:2:1 (Bayer arrangement), as illustrated in FIG.3D. Alternatively, the pixel number ratio (the ratio of light receivingarea) of red and green to blue may be 1:6:1.

Although the number of subpixels 112 provided in the pixel 111 may beone, two or more subpixels are preferably provided. For example, whentwo or more subpixels 112 sensing the same wavelength band are provided,the redundancy is increased, and the reliability of the imaging device100 can be increased.

When an infrared (IR) filter that transmits infrared light and absorbsor reflects visible light is used as the filter, the imaging device 100that senses infrared light can be fabricated.

Furthermore, when a neutral density (ND) filter (dark filter) is used asa filter 602, output saturation which occurs when a large amount oflight is incident on a photoelectric conversion element (light-receivingelement) can be prevented. With a combination of ND filters withdifferent dimming capabilities, the dynamic range of the imaging devicecan be increased.

Besides the above-described filter, the pixel 111 may be provided with alens. An arrangement example of the pixel 111, the filter 602, and alens 600 is described with cross-sectional views in FIGS. 4A and 4B.With the lens 600, the photoelectric conversion element can receiveincident light efficiently. Specifically, as illustrated in FIG. 4A,light 660 is incident on a photoelectric conversion element 220 throughthe lens 600, the filter 602 (a filter 602R, a filter 602G, and a filter602B), a pixel circuit 230, and the like formed in the pixel 111.

However, as illustrated in a region surrounded by a two-dot chain line,part of light 660 indicated by arrows may be blocked by part of a wiringlayer 604. Thus, a preferred structure is such that the lens 600 and thefilter 602 are provided on the photoelectric conversion element 220side, as illustrated in FIG. 4B, whereby the incident light can beefficiently received by the photoelectric conversion element 220. Whenthe light 660 is incident on the photoelectric conversion element 220from the photoelectric conversion element 220 side, the imaging device100 with high sensitivity can be provided.

As the photoelectric conversion element 220 illustrated in FIGS. 4A and4B, a photoelectric conversion element in which a PIN junction is formedmay be used. A photoelectric conversion element in which a PIN junctionis formed will be described in detail in Embodiment 3.

The photoelectric conversion element 220 may be formed using a materialcapable of generating electric charge by absorbing radiation. Examplesof a material capable of generating electrical charge by absorbingradiation include selenium, lead iodide, mercury iodine, galliumarsenide, CdTe, and CdZn.

The use of selenium for the photoelectric conversion element 220 enablesthe photoelectric conversion element 220 to have a favorable lightabsorption coefficient over a wide wavelength range including X-rays andgamma rays in addition to visible light, ultraviolet light, and infraredrays.

One pixel 111 included in the imaging device 100 may include a subpixelwith a first filter in addition to the subpixels illustrated in FIGS. 2Aand 2B and FIGS. 3A to 3D. An example is illustrated in FIGS. 10A and10B.

FIG. 10A shows an example in which the pixel 111 shown in FIGS. 2A and2B includes a subpixel provided with a filter 6021 (hereinafter referredto as subpixel 1121), in addition to the subpixel 112R, the subpixel112G, and the subpixel 112B. It is preferable that a filter thattransmits light with the wavelength band that the light P1 has beprovided as the filter 6021. In the case where light with an infraredwavelength band is used as the light P1, for example, a color filtertransmitting light with an infrared wavelength band can be used as thefilter 6021.

The cross-sectional view in FIG. 10B shows an example in which the pixel111 includes four subpixels 112: the subpixel 112R, the subpixel 112G,the subpixel 112B, and the subpixel 1121 provided with the filter 602R,the filter 602G, the filter 602B, and the filter 6021, respectively. Inaddition, the pixel 111 is provided with the lens 600.

[Circuit Configuration Example of Subpixel 112]

Next, a specific circuit configuration example of the subpixel 112 willbe described with reference to a circuit diagram in FIG. 5 . FIG. 5shows a circuit configuration example in which a subpixel 112[n] in thepixel 111 in an n-th row is electrically connected to a subpixel112[n+1] in the pixel 111 in an (n+1)-th row through a transistor 129.The transistor 129 can function as the switch 201, the switch 202, orthe switch 203.

FIG. 5 shows an example in which photodiodes are used as photoelectricconversion elements.

In the circuit diagram shown in FIG. 5 , specifically, the subpixel112[n] in the pixel 111 in the n-th row includes a photodiode PD[n](photoelectric conversion element), a transistor 121, a transistor 123,and a transistor 124. The subpixel 112[n+1] in the pixel 111 in the(n+1)-th row includes a photodiode PD[n+1], a transistor 125, atransistor 127, and a transistor 128.

In this embodiment, the case where n-channel transistors are used as thetransistors 121 to 129 will be described. Thus, each of the transistors121 to 129 has electrical conduction (in an on state) between a sourceand a drain when a signal supplied to a gate is at H potential, and thetransistors 121 to 129 have no electrical conduction (in an off state)when the signal is at L potential.

However, one embodiment of the present invention is not limited to theabove, and p-channel transistors can be used as the transistors 121 to129. Alternatively, n-channel transistors and p-channel transistors maybe used in combination as appropriate.

In the circuit configuration in FIG. 5 , one of an anode and a cathodeof the photodiode PD[n] is electrically connected to a wiring 131 fromwhich a potential VP can be supplied. The other of the anode and thecathode of the photodiode PD[n], one of a source and a drain of thetransistor 121, and one of a source and a drain of a transistor 122 areelectrically connected to a node ND[n]. The other of the source and thedrain of the transistor 122 is electrically connected to a wiring 133from which a potential VR can be supplied. A gate of the transistor 122is electrically connected to a wiring 141 from which a potential PR canbe supplied. The other of the source and the drain of the transistor 121and a gate of the transistor 123 are electrically connected to a nodeFD[n] which is a charge accumulation region. A gate of the transistor121 is electrically connected to a wiring 144[n] from which a potentialTX[n] can be supplied. One of a source and a drain of the transistor 123is electrically connected to a wiring 135 from which a potential VO canbe supplied, and the other of the source and the drain of the transistor123 is electrically connected to one of a source and a drain of thetransistor 124. The other of the source and the drain of the transistor124 is electrically connected to a wiring 137[m], and a gate of thetransistor 124 is electrically connected to a wiring 146[n] from which apotential SEL can be supplied. One of a source and a drain of thetransistor 129 is electrically connected to the node ND[n], and a gateof the transistor 129 is electrically connected to a wiring 142 fromwhich a potential PA can be supplied.

One of an anode and a cathode of the photodiode PD[n+1] is electricallyconnected to a wiring 132 from which the potential VP can be supplied.The other of the anode and the cathode of the photodiode PD[n+1], one ofa source and a drain of the transistor 125, and one of a source and adrain of a transistor 126 are electrically connected to a node ND[n+1]which is a charge accumulation region. The other of the source and thedrain of the transistor 126 is electrically connected to a wiring 134from which the potential VR can be supplied. A gate of the transistor126 is electrically connected to a wiring 143 from which the potentialPR can be supplied. The other of the source and the drain of thetransistor 125 and a gate of the transistor 127 are electricallyconnected to a node FD[n+1]. A gate of the transistor 125 iselectrically connected to a wiring 144[n+1] from which the potentialTX[n+1] can be supplied. One of a source and a drain of the transistor127 is electrically connected to a wiring 136 from which the potentialVO can be supplied. The other of the source and the drain of thetransistor 127 is electrically connected to one of a source and a drainof the transistor 128. The other of the source and the drain of thetransistor 128 is electrically connected to the wiring 137[m]. A gate ofthe transistor 128 is electrically connected to a wiring 146[n+1] fromwhich the potential SEL can be supplied. The other of the source and thedrain of the transistor 129 is electrically connected to the nodeND[n+1].

The wiring 131 and the wiring 132 may be one common wiring although theyare separate wirings in FIG. 5 . The wiring 141 and the wiring 143 maybe one common wiring although they are separate wirings in FIG. 5 . Thewiring 135 and the wiring 136 may be one common wiring although they areseparate wirings in FIG. 5 .

Note that although the photodiodes are used as photoelectric conversionelements in FIG. 5 , one embodiment of the present invention is notlimited thereto as long as elements capable of photoelectric conversionare used.

Operation Example 1

Next, an example of imaging operation for taking a two-dimensional imageby the imaging device 100 with a global shutter system will be describedwith reference to FIG. 6 , FIGS. 7A and 7B, FIGS. 8A and 8B, and FIGS.9A and 9B. The imaging with a global shutter system can be performed inthe following manner: a reset operation and an accumulation operationare concurrently performed in all of the subpixels 112, and a readingoperation is sequentially performed. As an operation example of thesubpixels 112, description will be made with reference to the subpixel112[n] and the subpixel 112[n+1].

FIG. 6 is a timing chart showing operation of the subpixels 112, andFIGS. 7A and 7B, FIGS. 8A and 8B, and FIGS. 9A and 9B are circuitdiagrams showing operation states of the subpixels 112. For easyunderstanding of the driving method, either the H potential or the Lpotential is supplied to the above wirings and nodes unless otherwisespecified, in the timing chart shown in this embodiment.

With a global shutter system, the accumulation operations of all pixels111 can be conducted during one period. Thus, unlike the case of using arolling shutter system, distortion of a taken image, which is caused bythe accumulation operations conducted in different periods, is notgenerated. Note that a frame period in the case of using the globalshutter system is referred to as a period 301 in FIG. 6 . The period 301corresponds to the sum of the lengths of time required for the resetoperation, the accumulation operation, and the operation of reading datafrom pixels in all rows. Further, the period 301 corresponds to a periodfrom the end of a reset operation to the start of the next resetoperation, for example.

In Operation Example 1, the imaging operation in the case where thepotential PA is set at L potential and the transistor 129 is in an offstate is described. When the potential PA is set at L potential, thesubpixel 112[n] and the subpixel 112[n+1] can be made to operateindependently. Furthermore, the potential VR is set at H potential, andthe potential VP and the potential VO are set at L potentials. Thepotential SEL[n] and the potential SEL[n+1] are set at L potentials.

[Reset Operation]

First, at a time T1, the potential PR, the potential TX[n], and thepotential TX[n+1] are set at H potentials. Then, the transistor 121 andthe transistor 122 are turned on, and the node ND[n] and the node FD[n]are set at H potentials. In addition, the transistor 125 and thetransistor 126 are turned on, and the node ND[n+1] and the node FD[n+1]are set at H potentials. Through the operation, the amount of chargeheld in the node FD[n] and the node FD[n+1] are reset (see FIG. 7A). Aperiod from the time T1 to a time T2 is referred to as a reset period.The operation during the reset period is referred to as a resetoperation.

Although not illustrated, in the reset period, all the nodes FD[n] andnodes FD[n+1] in the imaging device 100 are reset.

[Accumulation Operation]

Next, at the time T2, the potential PR is set at L potential. Thepotential TX[n] and the potential TX[n+1] remain at H potentials.Furthermore, at the time T2, the photodiode PD[n] and the photodiodePD[n+1] are supplied with a reverse bias. When light is incident on thephotodiode PD[n] and the photodiode PD[n+1] in a state where the reversebias is applied to the photodiode PD[n] and the photodiode PD[n+1],current flows from one of electrodes to the other electrode in each ofthe photodiode PD[n] and the photodiode PD[n+1] (see FIG. 7B). Theamount of current varies depending on the intensity of light in thisstate. In other words, as the intensity of light incident on thephotodiode PD[n] and the photodiode PD[n+1] is increased, the amount ofcurrent is increased, and the amount of charge flowing out from the nodeFD[n] and the node FD[n+1] is increased. In contrast, as the intensityof light incident on the photodiode PD[n] and the photodiode PD[n+1] islow, the amount of current is reduced, and the amount of charge flowingout from the node FD[n] and the node FD[n+1] is reduced. Thus, thehigher the intensity of light becomes, the larger the amount of changein the potential of the node FD[n] and the node FD[n+1] becomes; thelower the intensity of light becomes, the smaller the amount of changebecomes.

Next, at a time T3, the potential TX[n] and the potential TX[n+1] areset at L potentials, whereby the transistor 121 and the transistor 125are turned off. When the transistor 121 and the transistor 125 areturned off, the charge transfer from the node FD [n] and the nodeFD[n+1] to the photodiode PD[n] and the photodiode PD[n+1] stops, andthe potentials of the node FD[n] and the node FD[n+1] are determined(see FIG. 8A). A period from the time T2 to the time T3 is referred toas an exposure period. The exposure period in Operation Example 1 isreferred to as a period 311 in FIG. 6 . The operation during theexposure period is referred to as an accumulation operation.

[Reading Operation]

Next, at a time T4, the potential SEL supplied to the wiring 146[n] isset at H potential. Here, the case where n=1 (first row) is described.Immediately before the H potential is supplied to the wiring 146[n], thewiring 137[m] is pre-charged so that the potential becomes H potential.When the potential SEL supplied to the wiring 146[n] is at H potential,the transistor 124 is turned on, and the potential of the wiring 137[m]is lowered at a rate corresponding to the potential of the node FD[n](see FIG. 8B). At a time T5, the potential SEL supplied to the wiring146[n] is set at L potential, whereby the transistor 124 is turned off,and the potential of the wiring 137[m] is determined. The potential ofthe wiring 137[m] at this time is measured, so that the amount of lightreceived by the subpixel 112[n] can be calculated.

Next, at the time T5, the potential SEL supplied to the wiring 146[n+1](the wiring 146 in the second row in this case) is set at H potential.Immediately before the potential supplied to the wiring 146[n+1] is setat H potential, the wiring 137[m] is pre-charged so that the potentialbecomes H potential. When the potential SEL supplied to the wiring146[n+1] is set at H potential, the transistor 128 is turned on, and thepotential of the wiring 137[m] is lowered at a rate corresponding to thepotential of the node FD[n+1] (see FIG. 9A). At a time T6, the potentialSEL supplied to the wiring 146[n+1] is set at L potential, whereby thetransistor 128 is turned off, and the potential of the wiring 137[m] isdetermined (see FIG. 9B). The potential of the wiring 137[m] at thistime is measured, so that the amount of light received by the subpixel112[n+1] can be calculated.

After the time T6, the potential of the wiring 137[m] is measured inorder from the third row, whereby the potentials of the wirings 137[m]in the n-th row and the (n+1)-th row can be obtained. The potentials ofthe wirings 137[m] in the first to p-th rows are measured, whereby theamount of light received by the pixels 111 in the imaging device 100 canbe obtained. Thus, an image data of the object taken with the imagingdevice 100 can be obtained. For example, a period during which theamount of received light in each row is calculated, such as a periodfrom the time T4 to the time T5, is referred to as a reading period. Theoperation during the reading period is referred to as a readingoperation. Note that the timing of performing the reading operation canbe determined as appropriate. The potentials of wirings 137 in the firstto q-th columns, connected to the n-th row, may be measured in orderfrom the first column, measured concurrently from the first to q-thcolumns, or measured per unit of plural columns.

In the global shutter system, the reset operations are concurrentlyconducted in all pixels, and the accumulation operations areconcurrently conducted in all pixels. Thus, the potential TX[n], thepotential TX[n+1], and the potential PR of pixels in all columns may bechanged all at once.

A period from the end of the accumulation operation and the start of thereading operation, during which charge is held at the nodes FD in thepixels in each row, is referred to as a charge holding period. In theglobal shutter system, the reset operation and the accumulationoperation are performed on all pixels concurrently, and therefore, theexposure period ends in all pixels at substantially the same timing.However, the reading operation is sequentially performed on the pixelsper row, and thus the charge holding period differs between rows. Forexample, the charge holding period in the first row lies between thetime T3 and the time T4, and the charge holding period in the second rowlies between the time T3 and the time T5. The reading operation isperformed row by row; therefore, the timing of when the reading periodstarts varies from one row to another. Thus, the length of the chargeholding period in the last row is the longest.

When an image with a uniform grayscale level is obtained, output signalsin all the pixels ideally have potentials of the same level. However, inthe case where the length of the charge holding period varies from onepixel row to another, if charge accumulated at the nodes FD in thepixels in each row leaks out over time, the potential of an outputsignal varies from one row to another, and image data varies ingrayscale level from one row to another.

Thus, it is preferable for the transistor 121 and the transistor 125 touse a transistor with extremely low off-state current. With use of atransistor with extremely low off-state current for each of thetransistor 121 and the transistor 125, the amount of change inpotentials of the node FD[n] and the node FD[n+1] due to a difference inthe length of charge holding period can be small, even when the image istaken with the global shutter system. In that case, even when an imageis taken with the global shutter system, it is possible to suppressvariation in grayscale level of image data due to a difference in thelength of the charge holding period, and it is possible to enhance thequality of taken images.

In the case where the global shutter driving method is performed withuse of the circuit configuration in FIG. 5 , there is a possibility thatimage data of the pixels in the n-th row is mixed with image data of thepixels in the (n+1)-th row. Thus, it is preferable for the transistor129 to use a transistor with extremely low off-state current. With useof the transistor with extremely low off-state current for thetransistor 129, a mixture of the image data can be suppressed.

According to one embodiment of the present invention, the quality oftaken image can be enhanced.

Operation Example 2

Next, an example in which a three-dimensional image is taken with theimaging device 100 shown in FIGS. 1A and 1B will be described. Employingone embodiment of the present invention makes it possible to take atwo-dimensional image and a three-dimensional image with the imagingdevice 100.

For three-dimensional imaging, the circuit configuration example of thesubpixels 112 shown in FIG. 5 , which is used for two-dimensionalimaging, can be used, for example.

An operation example of the imaging device 100 having the circuitconfiguration shown in FIG. 5 and capable of three-dimensional imagingwill be described with reference to FIG. 12 , FIGS. 13A and 13B, FIGS.14A and 14B, FIGS. 15A and 15B, and FIGS. 16A and 16B. FIG. 12 is atiming chart showing operation of the subpixels 112, and FIGS. 13A and13B, FIGS. 14A and 14B, FIGS. 15A and 15B, and FIGS. 16A and 16B arecircuit diagrams showing operation states of the subpixels 112.

Note that a frame period in Operation Example 2 is referred to as aperiod 401 in FIG. 12 . The period 401 corresponds to the sum of lengthsof time required for the reset operation, the accumulation operation,and the operation of reading data from pixels in all rows. Further, theperiod 401 corresponds to a period from the end of a reset operation tothe start of the next reset operation.

In Operation Example 2, the imaging operation in the case where thepotential PA is set at H potential to turn the transistor 129 on isdescribed. In the case where the potential PA is set at H potential, thesubpixel 112[n] and the subpixel 112[n+1] can be used while beingconnected in parallel. Furthermore, the potential VR is set at Hpotential, and the potential VP and the potential VO are set at Lpotentials.

When the photodiode PD[n] in the subpixel 112[n] and the photodiodePD[n+1] in the subpixel 112[n+1] are used while being connected inparallel, a plurality of charge accumulation regions can be provided.The provision of the plurality of charge accumulation regions canachieve high light sensitivity. In addition, three-dimensional imagingin a short period of time becomes possible.

[Sensed Light]

In Operation Example 2, an example in which an object is irradiated withlight P1 and light P2 reflected from the object (hereinafter referred toas reflected light P2) is sensed such that a distance x between theimaging device 100 and the object is calculated will be described. Athree-dimensional image can be made with the use of the distance x.

FIG. 36 is a schematic diagram showing an example in which a surface ofan object 620 is irradiated with the light P1, which is emitted from thelight source 190 in the imaging device 100, and the reflected light P2is incident on the photodiode PD in the pixel 111(n,m) in the n-th rowand m-th column. In FIG. 36 , the imaging device 100 includes the lightsource 190, the pixel 111, and the lens 600. The imaging device 100 mayinclude two or more lenses. For example, the imaging device 100 mayinclude the lens 600 and a lens 610, as shown in FIG. 36 .

As the light P1, light with any wavelength can be used as long as it canbe converted by the photoelectric conversion element used in the imagingdevice 100. Visible light, ultraviolet light, infrared light, and thelike can be used, for example. X-rays or gamma rays may also be used asthe light P1. Light having a wide wavelength distribution such as whitelight may also be used.

The object 620 is sometimes irradiated with light other than the lightP1. Light with which the object 620 is irradiated, other than the lightP1, is referred to as outside light P3. Examples of the outside light P3include sunlight and lighting.

Here, the intensity of the light P1 is preferably higher than that ofthe outside light P3. Using light having a distribution of intensity ina specific wavelength range, such as infrared light, as the light P1 andproviding the photodiode PD with a filter that transmits light in thatwavelength range can reduce the influence of the outside light P3. Inthe case where fluorescent light is used as lighting, for example, theuse of infrared light as the sensed light can decrease the influence ofthe outside light P3 because fluorescent light includes little infraredlight. Thus, the quality of a three-dimensional image taken with theimaging device 100 can be enhanced.

It is preferable that the light P1 be capable of generating a short timepulse wave. A light emitting diode (LED) emitting pulse light may beused as the light P1, for example. Alternatively, pulse light may begenerated by opening and shutting a shutter, such as a technicalshutter, at high speed.

It is preferable that the object 620 be irradiated with the light P1widely and uniformly. Furthermore, the light P1 is preferably near thepixel 111.

[Reset Operation]

First, potentials of the potential PR, the potential TX[n], and thepotential TX[n+1] are set at H potentials at a time T1. Then, thetransistor 121 and the transistor 122 are turned on, and the node ND[n]and the node FD[n] are set at H potentials. In addition, the transistor125 and the transistor 126 are turned on, and the node ND[n+1] and thenode FD[n+1] are set at H potentials. Through the operation, the amountof charge accumulated at the node FD[n] and the node FD[n+1] are reset(see FIG. 13A).

Since the transistor 129 is in an on state in Operation Example 2,either the transistor 122 or the transistor 126 may be in an off stateduring the reset period. Although not illustrated, in the reset period,all the nodes FD [n] and nodes FD[n+1] in the imaging device 100 arereset.

[Accumulation Operation]

The photodiode PD[n] and the photodiode PD[n+1] are supplied with areverse bias. When light is incident on the photodiode PD[n] and thephotodiode PD[n+1] in a state where the reverse bias is applied to thephotodiode PD[n] and the photodiode PD[n+1], current flows from one ofelectrodes to the other electrode in each of the photodiode PD[n] andthe photodiode PD[n+1]. As described above, the amount of current atthis time changes depending on the light intensity. Thus, as theintensity of light is higher, the amount of change in potential of thenode FD[n] increases, and as the intensity of light is lower, the amountof change decreases.

At a time T2, the potential PR is set at L potential. The potentialTX[n+1] supplied to the wiring 144[n+1] is set at L potential. Thepotential TX[n] supplied to the wiring 144[n] remains at H potential(see FIG. 13B). Next, irradiation of the object 620 with the light P1 isstarted. The object 620 is irradiated with the light P1 during a period402 shown in FIG. 12 , i.e., from the time T2 to a time T4.

The light P1 with which the object 620 is irradiated at the time T2 isreflected by the surface of the object, and is incident on thephotodiode PD[n] and the photodiode PD[n+1] as the reflected light P2 ata time T3. Then, charge transfers from the node FD[n] to the photodiodePD[n] and the photodiode PD[n+1] (see FIG. 14A). Here, a period 403 inFIG. 12 corresponds to a period from the time T2 (the start of the lightP1 irradiation) to the time T3 (the incidence of the reflected light P2on the photodiodes).

The distance x between the imaging device 100 and the object will bedescribed. As shown in FIG. 36 , the distance between the light sourceof the imaging device 100 (i.e., the source of light P1) and a portionof the object 620 which is sensed by the pixel in the n-th row and them-th column is x1. The distance between the portion of the object 620which is sensed by the pixel in the n-th row and the m-th column and thepixel in the n-th row and the m-th column is x2. The distance x betweenthe imaging device 100 and the object 620 may be the average of x1 andx2, i.e., x may be equal to (x1+x2)/2. In the case where the distance x1and the distance x2 are sufficiently greater than the size of theimaging device 100, the distance x to the object 620 can be approximatedby x1 or x2.

Here, ΔT_(X)=2x/c is satisfied, where the length of the period 403,i.e., a difference between the time T2 and the time T3 is ΔT_(X), thedistance between the imaging device 100 and the object 620 is x, andlight speed is c. ΔT_(X) changes in accordance with the distance betweenthe imaging device and the object 620.

Then, at a time T4, the light P1 irradiation is stopped. In addition,the potential TX[n] supplied to the wiring 144[n] is set at L potential,and the potential TX[n+1] supplied to the wiring 144[n+1] is set at Hpotential (see FIG. 14B). When the potential TX[n+1] is set at Hpotential, the transistor 125 is turned on. Thus, charge transfers fromthe node FD[n+1] to the photodiodes PD[n] and PD[n+1]. When thepotential TX[n] is set at L potential, the transistor 121 is turned off.When the transistor 121 is turned off, the charge transfer from the nodeFD [n] to the photodiodes PD[n] and PD[n+1] is stopped. The potential ofthe node FD[n] depends on the amount of light received by thephotodiodes PD[n] and PD[n+1] in a period during which the transistor121 is on, i.e., a period from the time T2 to the time T4.

The charge transfer from the node FD[n] due to the reflected light P2 ofthe light P1 being incident on the photodiodes PD[n] and PD[n+1] occursin a period 405, i.e., a period from the time T3 to the time T4 shown inFIG. 12 . The period 405 is referred to as a first exposure period.

When the object 620 is irradiated with the outside light P3 and thereflected light is incident on the photodiodes PD[n] and PD[n+1] in aperiod from the time T2 to the time T4, charge transfers to the nodeFD[n] to affect the potential of the node FD[n].

It is preferable that the light P1 is more intense than the outsidelight P3. As long as the light P1 is sufficiently more intense than theoutside light P3, the amount of the charge transfer from the node FD[n]in the period from the time T2 to the time T4 is determined by theamount of the charge transfer due to the reflected light P2 beingincident on the photodiodes PD[n] and PD[n+1] in the period 405 in mostcases.

For canceling the effect of the outside light P3, the background may bemeasured beforehand in a state where the light P1 is not emitted, toperform adjustment.

The incidence of the reflected light P2 that has been incident on thephotodiodes PD[n] and PD[n+1] from the time T3 is stopped at a time T5.

The time T5 depends on the distance between the imaging device and theobject 620. Further, T5=T3+ΔT_(A) is satisfied where the length of theperiod 402, i.e., a difference between the time T4 and the time T2 isΔT_(A).

Next, the potential TX[n+1] supplied to the wiring 144[n+1] is set at Lpotential at a time T6. The potential TX[n] remains at L potential (seeFIG. 15A). Charge can transfer from the node FD[n+1] to the photodiodesPD[n] and PD[n+1] during a period in which the transistor 125 is on,i.e., a period from the time T4 to the time T6. The potential of thenode FD[n+1] depends on the amount of light received by the photodiodesPD[n] and PD[n+1] during the period from the time T4 to the time T6.

The charge transfer from the node FD[n+1] due to the reflected light P2of the light P1 being incident on the photodiodes PD[n] and PD[n+1]occurs in a period 406, i.e., a period from the time T4 to the time T5shown in FIG. 12 . The period 406 is referred to as a second exposureperiod. The length of the period 404 shown in FIG. 12 , i.e., adifference between the time T5 and the time T3 is ΔT_(A).

When the object 620 is irradiated with the outside light P3 and thereflected light is incident on the photodiodes PD[n] and PD[n+1] in theperiod from the time T4 to the time T6, charge transfers to the nodeFD[n+1] to affect the potential of the node FD[n+1].

The length of the period 405, i.e., the length of the first exposureperiod, can be expressed as ΔT_(A)−ΔT_(X). The length of the period 406,i.e., the length of the second exposure period, is ΔT_(X).

As the distance x between the imaging device and the object 620increases, ΔT_(X) also increases. As ΔT_(X) increases, the firstexposure period becomes shorter and the second exposure period becomeslonger. That is, the amount of charge accumulated in the node FD[n]decreases and the amount of charge accumulated in the node FD[n+1]increases.

The accumulation operation in the (n+1)-th row is performed withoutperformance of the reset operation after the accumulation operation inthe n-th row, so that the frame period can be shortened.

For the subpixel 112[n], the subpixel 112[n+1], the transistor 129, andwirings connecting these elements, it is preferable that parasiticcapacitance formed in regions other than those forming the capacitanceof the nodes FD[n] and FD[n+1] (the parasitic capacitance is referred toas first parasitic capacitance, here) be sufficiently small. Examples ofthe first parasitic capacitance includes: capacitance formed by a wiringbetween the transistor 121 and the node ND[n] and another wiring or asemiconductor layer overlapping therewith; capacitance formed by awiring and a transistor between the node ND[n] and the node ND[n+1] andanother wiring or a semiconductor layer overlapping therewith;capacitance formed by a wiring between the node ND[n+1] and thetransistor 125 and another wiring or a semiconductor layer overlappingtherewith; and capacitance formed by a wiring between the node ND andthe photodiode PD and another wiring or a semiconductor layeroverlapping therewith.

The potential of the node FD[n+1] is determined by the reset operationat the time T1 and held until the time T4. Then, at the time T4, thepotential of the first parasitic capacitance, e.g., the potentials ofthe node ND[n] and the node ND[n+1], becomes the potential equivalent tothe potential of the node FD[n]. In the case where the first parasiticcapacitance is not sufficiently smaller than the capacitance of the nodeFD[n+1], a significant voltage drop in the node FD[n+1] may occur rightafter the transistor 125 is turned on at the time T4. In such a case, asecond reset operation may be performed right after the transistor 125is turned on to reset the potential of the node FD[n+1] at H potential,which will be described in Operation Example 3 later.

[Reading Operation]

First, at a time T7, the potential SEL supplied to the wiring 146[n] isset at H potential. Here, the case where n ισ 1 (first row) isdescribed. Immediately before the H potential is supplied to the wiring146[n], the wiring 137[m] is pre-charged so that the potential becomes Hpotential. When the potential SEL supplied to the wiring 146[n] is at Hpotential, the transistor 124 is turned on, and the potential of thewiring 137[m] is lowered at a rate corresponding to the potential of thenode FD[n] (see FIG. 15B). At a time T8, the potential SEL supplied tothe wiring 146[n] is set at L potential, whereby the transistor 124 isturned off, and the potential of the wiring 137[m] is determined. Thepotential of the wiring 137[m] at this time is measured, so that theamount of light received by the subpixel 112[n] can be calculated.

Next, at the time T8, the potential SEL supplied to the wiring 146[n+1](the wiring 146 in the second row in this case) is set at H potential.Immediately before the potential supplied to the wiring 146[n+1] is setat H potential, the wiring 137[m] is pre-charged so that the potentialbecomes H potential. When the potential SEL supplied to the wiring146[n+1] is set at H potential, the transistor 128 is turned on, and thepotential of the wiring 137[m] is lowered at a rate corresponding to thepotential of the node FD[n+1] (see FIG. 16A). At the time T6, thepotential SEL supplied to the wiring 146[n+1] is set at L potential,whereby the transistor 128 is turned off, and the potential of thewiring 137[m] is determined (see FIG. 16B). The potential of the wiring137[m] at this time is measured, so that the amount of light received bythe subpixel 112[n+1] can be calculated.

After a time T9, the potential of the wiring 137[m] is measured in orderfrom the third row, whereby the potentials of the wirings 137[m] in then-th row and the (n+1)-th row can be obtained. The potentials of thewirings 137[m] in the first to p-th rows are measured, whereby theamount of light received by the pixels 111 in the imaging device 100 canbe obtained. Thus, image data of the object taken with the imagingdevice 100 can be obtained. For example, a period during which theamount of received light in each row is calculated, such as a periodfrom the time T7 to the time T8, is referred to as a reading period. Theoperation during the reading period is referred to as a readingoperation. Note that the timing of performing the reading operation canbe determined as appropriate. The potentials of wirings 137 in the firstto q-th columns, connected to the n-th row, may be measured in orderfrom the first column, measured concurrently from the first to q-thcolumns, or measured per unit of plural columns.

Here, the potential of the n-th row and the potential of the (n+1)-throw obtained by the reading operation are S[n] and S[n+1], respectively.In the case where the light P1 is sufficiently more intense than theoutside light P3, it is assumed that S[n] and S[n+1] can be approximatedby k(ΔT_(A)−ΔT_(X)) and k·ΔT_(X), respectively. Here, k is a constant.ΔT_(X) can be obtained by the following formula:ΔT_(X)=ΔT_(A)·S[n+1]/(S[n]+S[n+1]). Further, x can be obtained by thefollowing formula: x={cΔT_(A)·S[n+1]/(S[n]+S[n+1])}/2. With the use ofthe distance x which is obtained by the calculation, a three-dimensionalimage can be made.

The reset operation and the accumulation operation may be performed in acondition where the wirings 144[n] are in the odd-numbered rows and thewirings 144[n+1] is in the even-numbered rows. The number of pixels inwhich the other electrodes of the photodiodes PD are shared isincreased, whereby the frequency of continuous accumulation operationscan be increased. In other words, image data of continuous frames can beobtained with m short intervals in the following manner; in m pixels,the other electrodes of the photodiodes are shared with each other; thecharge is sequentially accumulated in charge accumulation regions in thepixels through the continuous accumulation operations performed m times;and the taken image data in the pixels is sequentially read out.

According to one embodiment of the present invention, a solid-stateimaging device with which three-dimensional images can be taken with ashort duration can be provided. In addition, a solid-state imagingdevice capable of high-definition two-dimensional imaging can beprovided.

[Modification Example of Operation Example 2]

FIG. 5 shows a configuration in which the subpixel 112[n] and thesubpixel 112[n+1] are arranged in adjacent rows and connected to eachother in parallel. A configuration in which a subpixel 112[m] and asubpixel 112[m+1] are arranged in adjacent columns and connected to eachother in parallel may also be employed.

Further, in the case where the subpixel 112[m] and the subpixel 112[m+1]are arranged in adjacent columns and connected to each other inparallel, the wiring 146[n] and the wiring 146[n+1] may be one commonwiring and an individual OUT[m] may be provided for each of thesubpixels 112[m] and 112[m+1], as shown in FIG. 17 .

In the circuit configuration in FIG. 17 , one of an anode and a cathodeof a photodiode PD[m] is electrically connected to a wiring 131 fromwhich a potential VP can be supplied. The other of the anode and thecathode of the photodiode PD[m], one of a source and a drain of atransistor 121, and one of a source and a drain of a transistor 122 areelectrically connected to a node ND[m]. The other of the source and thedrain of the transistor 122 is electrically connected to a wiring 133from which a potential VR can be supplied. A gate of the transistor 122is electrically connected to a wiring 141 from which a potential PR canbe supplied. The other of the source and the drain of the transistor 121and a gate of a transistor 123 are electrically connected to a nodeFD[m]. A gate of the transistor 121 is electrically connected to awiring 144[m] from which a potential TX[m] can be supplied. One of asource and a drain of the transistor 123 is electrically connected to awiring 135 from which a potential VO can be supplied, and the other ofthe source and the drain of the transistor 123 is electrically connectedto one of a source and a drain of a transistor 124. The other of thesource and the drain of the transistor 124 is electrically connected toa wiring 137[m], and a gate of the transistor 124 is electricallyconnected to a wiring 146[n] from which a potential SEL can be supplied.One of a source and a drain of the transistor 169 is electricallyconnected to the node ND[m], and a gate of the transistor 169 iselectrically connected to a wiring 182 from which a potential PA can besupplied.

In the circuit configuration in FIG. 17 , one of an anode and a cathodeof a photodiode PD[m+1] is electrically connected to a wiring 172 fromwhich the potential VP can be supplied. The other of the anode and thecathode of the photodiode PD[m+1], one of a source and a drain of atransistor 165, and one of a source and a drain of a transistor 166 areelectrically connected to a node ND[m+1]. The other of the source andthe drain of the transistor 166 is electrically connected to a wiring174 from which the potential VR can be supplied. A gate of thetransistor 166 is electrically connected to a wiring 183 from which thepotential PR can be supplied. The other of the source and the drain ofthe transistor 165 and a gate of a transistor 167 are electricallyconnected to a node FD[m+1]. A gate of the transistor 165 iselectrically connected to a wiring 144[m+1] from which the potentialTX[m+1] can be supplied. One of a source and a drain of a transistor 167is electrically connected to a wiring 176 from which the potential VOcan be supplied. The other of the source and the drain of the transistor167 is electrically connected to one of a source and a drain of atransistor 168. The other of the source and the drain of the transistor168 is electrically connected to a wiring 137[m+1], and a gate of thetransistor 168 is electrically connected to the wiring 146[n] from whichthe potential SEL can be supplied. The other of the source and the drainof the transistor 169 is electrically connected to the node ND[m+1].

The wiring 131 and the wiring 172 may be one common wiring although theyare separate wirings in FIG. 17 . The wiring 141 and the wiring 183 maybe one common wiring although they are separate wirings in FIG. 17 . Thewiring 135 and the wiring 176 may be one common wiring although they areseparate wirings in FIG. 17 .

An example of an imaging operation of the imaging device 100 having theconfiguration in FIG. 17 is shown in FIG. 18 . A period 501 correspondsto the sum of lengths of time required for the reset operation, theaccumulation operation, and the operation of reading data from pixels inall rows. Further, the period 501 corresponds to a period from the endof a reset operation to the start of the next reset operation, forexample.

[Reset Operation]

First, at a time T1, the potential PR, the potential TX[m], and thepotential TX[m+1] are set at H potentials, and the amount of charge heldin each of the node FD[m] and the node FD[m+1] is reset. Although notillustrated, in the reset period, all the nodes FD[m] and nodes FD[m+1]in the imaging device 100 are reset.

[Accumulation Operation]

Next, at a time T2, the potential PR is set at L potential. Thepotential TX[m+1] supplied to the wiring 144[m+1] is set at L potential.The potential TX[m] supplied to the wiring 144[m] remains at Hpotential. In addition, the object 620 starts to be irradiated with thelight P1.

The light P1 with which the object 620 is irradiated at the time T2 isreflected by the surface of the object, and is incident on thephotodiode PD[m] and the photodiode PD[m+1] as the reflected light P2 ata time T3.

Then, the light P1 irradiation is stopped at a time T4. The potentialTX[m] supplied to the wiring 144[m] is set at L potential, and thepotential TX[m+1] supplied to the wiring 144[m+1] is set at H potential.The charge transfer caused by the reflected light P2 of the light P1being incident on the photodiode PD[m] and the photodiode PD[m+1] occursduring a period from the time T3 to the time T4.

The reflected light P2 that is incident on and received by thephotodiodes PD[m] and PD[m+1] at the time T3 keeps being received bythem until a time T5.

Next, at a time T6, the potential TX[m+1] supplied to the wiring144[m+1] is set at L potential. The potential TX[m] remains at Lpotential. The charge transfer caused by the reflected light P2 of thelight P1 being incident on the photodiode PD[m] and the photodiodePD[m+1] occurs during a period from the time T4 to the time T5.

For example, in the configuration shown in FIG. 17 , the reset operationand the accumulation operation may be performed in a condition where thewirings 144[m] are in the odd-numbered columns and the wirings 144[m+1]are in the even-numbered columns.

[Reading Operation]

For reading, a method similar to the reading method described inOperation Example 2 can be used.

Another example of a reading method will be described. Here, thepotential VO is at L potential. The potential SEL supplied to the wiring146[n] is set at H potential to perform reading. It is preferable that,immediately before H potential is supplied to the wiring 146[n], thewiring 137[m] and the wiring 137[m+1] are pre-charged so that thepotentials become H potentials. In the case where the wiring 137[m] andthe wiring 137[m+1] are pulled up with the use of a resistor or atransistor, precharge is not necessary. At this time, the potentials ofthe wiring 137[m] and the wiring 137[m+1] change in accordance with thepotentials of the node ND[m] and the node ND[m+1]. Therefore, the amountof light received by the subpixel 112[m] and the subpixel 112[m+1] canbe calculated by sequentially measuring the potentials of the wiring137[m] and the wiring 137[m+1]. Employing this reading method canincrease the reading speed.

Another example of a reading method different from the above will bedescribed. Here, the potential VO is at H potential. The wiring 137[m]and the wiring 137[m+1] are pulled up with the use of a resistor or atransistor. The potential SEL supplied to the wiring 146[n] is set at Hpotential to perform reading. At this time, the potentials of the wiring137[m] and the wiring 137[m+1] change in accordance with the potentialsof the node ND[m] and the node ND[m+1]. Therefore, the amount of lightreceived by the subpixel 112[m] and the subpixel 112[m+1] can becalculated by sequentially measuring the potentials of the wiring 137[m]and the wiring 137[m+1]. In this reading method, a change in thepotentials of the wiring 137[m] and the wiring 137[m+1] is approximatelyproportional to the potentials of the node ND[m] and the node ND[m+1];thus, the accuracy of distance calculation can be improved.

Here, the potential in the n-th row and the m-th column and thepotential in the n-th row and the (m+1)-th column obtained by thereading operation are S[m] and S[m+1], respectively. In the case wherethe light P1 is sufficiently more intense than the outside light P3, adistance x between the imaging device 100 and the object 620 can beapproximated by {cΔT_(A)·S[m+1]/(S[m]+S[m+1])}/2. With the use of thedistance x obtained by the calculation, a three-dimensional image can bemade.

Operation Example 3

FIG. 19 is a timing chart showing an example of a method forthree-dimensional imaging. Immediately after the potential of the nodeFD[n] is determined and the transistor 125 is turned on, a second resetoperation is performed, in the method shown in FIG. 19 . Whether or notthe second reset operation is performed is a difference between FIG. 12and FIG. 19 .

An imaging operation of the imaging device 100 will be described belowwith reference to FIG. 19 .

[Reset Operation]

At a time T1, the potential PR and the potential TX[n] are set at Hpotentials. Then, the transistor 121 and the transistor 122 are turnedon, and the node ND[n] and the node FD[n] are set at H potentials.Through the operation, the amount of charge held at the node FD[n] isreset. Further, the transistor 126 is turned on. Next, the object 620starts to be irradiated with the light P1. The object 620 is irradiatedwith the light P1 during a period 502 and a period 503, i.e., during aperiod from the time T1 to a time T4 in FIG. 19 .

[Accumulation Operation]

The photodiode PD[n] and the photodiode PD[n+1] are supplied with areverse bias. At a time T2 after ΔT_(B) of the time T1, the potential PRis set at L potential. The potential TX[n] supplied to the wiring 144[n]remains at H potential. The potential TX[n+1] supplied to the wiring144[n+1] remains at L potential.

The light P1 with which the object 620 is irradiated at the time T2 isreflected by the surface of the object, and is incident on thephotodiode PD[n] and the photodiode PD[n+1] as the reflected light P2 atthe time T3. Then, charge transfers from the node FD[n] to thephotodiode PD[n] and the photodiode PD[n+1]. Here, a period 506 in FIG.19 corresponds to a period from the time T2 (the start of the light P1irradiation) to a time T3 (the incidence of the reflected light P2 onthe photodiodes).

Here, ΔT_(Y)=2x/c is satisfied where the length of the period 506, i.e.,a difference between the time T3 and the time T1, is ΔT_(Y), a distancebetween the imaging device and the object 620 is y, and the light speedis c, and ΔT_(Y) depends on the distance between the imaging device andthe object 620.

The charge transfer from the node FD[n] due to the reflected light P2 ofthe light P1 being incident on the photodiodes PD[n] and PD[n+1] occursin a period 507, i.e., a period from the time T3 to the time T4 shown inFIG. 19 . The length of the period 507 can be expressed as(ΔT_(B)+ΔT_(C)−ΔT_(Y)). The period 507 is referred to as a firstexposure period.

Next, the light P1 irradiation is stopped at a time T4. In addition, thepotential TX[n] supplied to the wiring 144[n] is set at L potential, andthe potential PR and the potential TX[n+1] supplied to the wiring144[n+1] are set at H potentials. Through the operation, the transistor125 and the transistor 126 are turned on and the node ND[n+1] is set atH potential. Through the operation, the amount of charge held at thenode ND[n+1] is reset. Further, the potential TX[n] set at L potentialturns off the transistor 121. The transistor 121 being turned off stopsthe charge transfer from the node FD[n] to the photodiodes PD[n] andPD[n+1], whereby the potential of the node FD[n] is determined.

Next, the potential PR is set at L potential at a time T5 after ΔT_(B)of the time T4. The potential TX[n+1] remains at H potential. Throughthe operation, the transistor 126 is turned off. In addition, chargetransfers from the node FD[n+1] to the photodiodes PD[n] and PD[n+1].The charge transfer from the node FD[n+1] caused by the reflected lightP2 of the light P1 being incident on the photodiode PD[n] and thephotodiode PD[n+1] occurs during a period 508, i.e., during a periodfrom the time T5 to a time T6 in FIG. 19 . The length of the period 508can be expressed as (ΔT_(Y)−ΔT_(B)). The period 508 is referred to as asecond exposure period.

Next, the potential TX[n+1] supplied to the wiring 144[n+1] is set at Lpotential at a time T7. The potential TX[n] remains at L potential. Thecharge transfer from the node FD[n+1] to the photodiodes PD[n] andPD[n+1] stops, and the potential of the node FD[n+1] is determined.

[Reading Operation]

A reading operation can be performed in a manner similar to thatdescribed in Operation Example 2.

The potential of the n-th row and the potential of the (n+1)-th rowobtained by the reading operation are S[n] and S[n+1], respectively. Inthe case where the light P1 is sufficiently more intense than theoutside light P3, it is assumed that S[n] and S[n+1] can be approximatedby j(ΔT_(C)+ΔT_(B)−ΔT_(Y)) and j(ΔT_(Y)−ΔT_(B)), respectively. Here, jis a constant. ΔT_(Y) can be obtained by the following formula:ΔT_(Y)={S[n+1]/(S[n]+S[n+1])}−ΔT_(C)+ΔT_(B). Further, y can be obtainedby the following formula: y=c·[{S[n+1]/(S[n]+S[n+1])}·ΔT_(C)+ΔT_(B)]/2.With the use of the distance y obtained by the calculation, athree-dimensional image can be made.

Embodiment 2

In this embodiment, another example of circuit configuration of thesubpixel 112 will be described with reference to drawings.

One of an anode and a cathode of the photodiode PD in the subpixel 112may be electrically connected to the node ND, and the other of the anodeand the cathode may be electrically connected to the wiring 131 (or thewiring 132) (see FIG. 20A). In this case, the potential VR is set at Lpotential, and the potential VP is set at H potential, whereby theimaging device 100 can be made to operate as in the above operationexample.

In addition, a capacitor 151 may be provided at the node FD in thesubpixel 112 (see FIG. 20B). With the capacitor 151, the data holdingtime of image data at the node FD can be increased. Furthermore, thedynamic range of the imaging device 100 can be increased.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Embodiment 3

In this embodiment, an example in which the imaging device 100 includesa CMOS image sensor that is a type of solid-state image sensor will bedescribed with reference to FIG. 21, FIG. 22 , FIG. 23 , and FIGS. 24Aand 24B. A pixel region 251 shown in FIG. 21 as a cross-sectional viewcorresponds to part of the pixel 111 in the imaging device 100. Aperipheral circuit region 252 shown in FIG. 21 as a cross-sectional viewcorresponds to part of a peripheral circuit in the imaging device 100.FIG. 22 is an enlarged view of a transistor 241 in FIG. 21 . FIG. 24A isan enlarged view of a transistor 281 in FIG. 21 . FIG. 24B is anenlarged view of a transistor 282 in FIG. 21 .

The imaging device 100 described in this embodiment includes aninsulating layer 102 over a substrate 101, and a photoelectricconversion element 220 having a PIN junction over the insulating layer102. The photoelectric conversion element 220 includes a p-typesemiconductor layer 221, an i-type semiconductor layer 222, and ann-type semiconductor layer 223. The photoelectric conversion element 220has a structure in which the i-type semiconductor layer 222 issandwiched between the p-type semiconductor layer 221 and the n-typesemiconductor layer 223. Note that the photoelectric conversion element220 may be formed with the p-type semiconductor layer 221 and the n-typesemiconductor layer 223 without the i-type semiconductor layer 222. Whenthe i-type semiconductor layer 222 is provided in the photoelectricconversion element 220, the photosensitivity can be increased. Thephotoelectric conversion element 220 described in this embodiment canfunction as the photodiode PD described in the above embodiment.

Note that an intrinsic semiconductor (i-type semiconductor) is ideally asemiconductor which does not include impurities and whose Fermi levellies substantially in the middle of the band gap, but in thisspecification and the like, a semiconductor to which an impurity servingas a donor or an impurity serving as an acceptor is added and whoseFermi level lies substantially in the middle of the band gap is alsoincluded in the category of the intrinsic semiconductor. Even when asemiconductor includes an impurity serving as a donor or an impurityserving as an acceptor, the semiconductor is included in the category ofintrinsic semiconductors as long as the semiconductor can function as anintrinsic semiconductor.

As the substrate 101, a glass substrate, a quartz substrate, a sapphiresubstrate, a ceramic substrate, a metal substrate, a semiconductorsubstrate, or the like can be used. Alternatively, a plastic substratehaving heat resistance to the processing temperature of this embodimentmay be used. Examples of the substrate include a semiconductor substrate(e.g., a single crystal substrate or a silicon substrate), a silicon oninsulator (SOI) substrate, a glass substrate, a quartz substrate, aplastic substrate, a metal substrate, a stainless steel substrate, asubstrate including stainless steel foil, a tungsten substrate, and asubstrate including tungsten foil. As an example of a glass substrate, abarium borosilicate glass substrate, an aluminoborosilicate glasssubstrate, a soda lime glass substrate, or the like can be given.

Further, a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate may be used as the substrate. Fabrication of thephotoelectric conversion element 220 with the use of a single crystalsemiconductor substrate or a polycrystalline semiconductor substrate canimprove the light detection sensitivity of the photoelectric conversionelement 220. In such cases, the formation of the i-type semiconductorlayer 222 may be omitted.

After the photoelectric conversion element 220 and the pixel circuit 230are formed, the substrate 101 may be removed by a mechanical polishingmethod, an etching method, or the like. In the case where the substrate101 is left, a material that transmits light sensed by the photoelectricconversion element 220 may be used for the substrate 101.

The insulating layer 102 can be formed to have a single-layer structureor a multi-layer structure using an oxide material such as aluminumoxide, magnesium oxide, silicon oxide, silicon oxynitride, galliumoxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, or tantalum oxide; a nitride materialsuch as silicon nitride, silicon nitride oxide, aluminum nitride, oraluminum nitride oxide; or the like. The insulating layer 102 can beformed by a sputtering method, a chemical vapor deposition (CVD) method,a thermal oxidation method, a coating method, a printing method, or thelike.

The p-type semiconductor layer 221, the i-type semiconductor layer 222,and the n-type semiconductor layer 223 may be formed, for example, insuch a manner that the i-type semiconductor layer 222 having an islandshape is formed over the insulating layer 102, and then a mask is formedover the i-type semiconductor layer 222, and impurity elements areselectively introduced into part of the i-type semiconductor layer 222.The impurity elements can be introduced by an ion implantation method,in which mass separation is performed, or an ion doping method, in whichmass separation is not performed. The mask is removed after the impurityelements are introduced.

The p-type semiconductor layer 221, the i-type semiconductor layer 222,and the n-type semiconductor layer 223 can be formed using a singlecrystal semiconductor, a polycrystalline semiconductor, amicrocrystalline semiconductor, a nanocrystal semiconductor, asemi-amorphous semiconductor, an amorphous semiconductor, or the like.For example, amorphous silicon or microcrystalline germanium can beused. Alternatively, a compound semiconductor such as silicon carbide orgallium arsenide can be used.

In the case where silicon is used as a material for formation of thep-type semiconductor layer 221, the i-type semiconductor layer 222, andthe n-type semiconductor layer 223, Group 13 elements can be used, forexample, as a p-type impurity element. As an n-type impurity element,for example, a Group 15 element can be used.

In the case where the semiconductor layer is formed using SOI, forexample, the insulating layer 102 may be a buried oxide (BOX) layer.

The imaging device 100 described in this embodiment includes aninsulating layer 103 and an insulating layer 104 over the p-typesemiconductor layer 221, the i-type semiconductor layer 222, and then-type semiconductor layer 223. The insulating layer 103 and theinsulating layer 104 can be formed using a material and a method similarto those of the insulating layer 102. Note that one of the insulatinglayers 103 and 104 may be omitted or another insulating layer may bestacked thereover.

Further, in the imaging device 100 described in this embodiment, aninsulating layer 105 having a flat surface is formed over the insulatinglayer 104. The insulating layer 105 can be formed using a material and amethod similar to those of the insulating layer 102. It is also possibleto use a low-dielectric constant material (a low-k material), asiloxane-based resin, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), or the like for the insulating layer 105. Further, thesurface of the insulating layer 105 may be subjected to chemicalmechanical polishing (CMP) treatment (hereinafter also referred to asCMP treatment). The CMP treatment can reduce unevenness of the surface,whereby coverage with an insulating layer or a conductive layer to beformed later can be increased.

In a region including the insulating layers 103 to 105 which overlapswith the p-type semiconductor layer 221, an opening 224 is formed, andin a region including the insulating layers 103 to 105 which overlapswith the n-type semiconductor layer 223, an opening 225 is formed.Contact plugs 106 are formed in the opening 224 and the opening 225. Thecontact plugs 106 are formed by filling the openings provided in theinsulating layers with a conductive material. As the conductivematerial, for example, a conductive material with high embeddability,such as tungsten or polysilicon, can be used. Although not illustrated,the side surface and the bottom surface of the material can be coveredwith a barrier layer (a diffusion prevention layer) such as a titaniumlayer, a titanium nitride layer, or a stack of these layers. In thiscase, the barrier film is regarded as part of the contact plug.

Over the insulating layer 105, an electrode 226 and an electrode 227 areformed. The electrode 226 is electrically connected to the p-typesemiconductor layer 221 via the contact plug 106 in the opening 224. Theelectrode 227 is electrically connected to the n-type semiconductorlayer 223 via the contact plug 106 in the opening 225.

Further, an insulating layer 107 is formed to cover the electrode 226and the electrode 227. The insulating layer 107 can be formed using amaterial and a method that are similar to those of the insulating layer105. A surface of the insulating layer 107 may be subjected to CMPtreatment. The CMP treatment can reduce unevenness of the surface,whereby coverage with an insulating layer or a conductive layer to beformed later can be increased.

The electrode 226 and the electrode 227 can be formed with asingle-layer structure or a stacked-layer structure using any of metalssuch as aluminum, titanium, chromium, nickel, copper, yttrium,zirconium, molybdenum, manganese, silver, tantalum, and tungsten, or analloy containing any of these metals as its main component. For example,a single-layer structure of a copper film containing manganese; atwo-layer structure in which an aluminum film is stacked over a titaniumfilm; a two-layer structure in which an aluminum film is stacked over atungsten film; a two-layer structure in which a copper film is stackedover a copper-magnesium-aluminum alloy film; a two-layer structure inwhich a copper film is stacked over a titanium film; a two-layerstructure in which a copper film is stacked over a tungsten film; athree-layer structure in which a titanium film or a titanium nitridefilm, an aluminum film or a copper film, and a titanium film or atitanium nitride film are stacked in this order; a three-layer structurein which a molybdenum film or a molybdenum nitride film, an aluminumfilm or a copper film, and a molybdenum film or a molybdenum nitridefilm are stacked in this order; a three-layer structure in which atungsten film, a copper film, and a tungsten film are stacked in thisorder; and the like can be given. Alternatively, an alloy film or anitride film in which aluminum and one or more elements selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium are combined may be used.

Note that a conductive material containing oxygen such as indium tinoxide, zinc oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon oxide is added, or a conductive materialcontaining nitrogen such as titanium nitride or tantalum nitride may beused. It is also possible to use a stacked-layer structure formed usinga material containing the above metal element and conductive materialcontaining oxygen. It is also possible to use a stacked-layer structureformed using a material containing the above metal element andconductive material containing nitrogen. It is also possible to use astacked-layer structure formed using a material containing the abovemetal element, conductive material containing oxygen, and conductivematerial containing nitrogen.

The photoelectric conversion element 220 senses the light 660 incidenton the insulating layer 102 side.

The transistor included in the pixel 111 may be provided to overlap withthe photoelectric conversion element. In FIG. 21 , the transistor 241and a transistor 246 are provided above the photoelectric conversionelement 220. Specifically, the transistor 241 and the transistor 246 areformed over the insulating layer 107 with an insulating layer 108 and aninsulating layer 109 positioned therebetween. Further, in FIG. 21 , atransistor 289 is provided above the transistor 281.

In this embodiment, the transistor 241, the transistor 246 and thetransistor 289 are each a top-gate transistor; however, a bottom-gatetransistor may be employed.

Alternatively, an inverted staggered transistor or a forward staggeredtransistor can also be used as the transistors. It is also possible touse a dual-gate transistor, in which a semiconductor layer in which achannel is formed is positioned between two gate electrodes. Further,the transistor is not limited to a transistor having a single-gatestructure; a multi-gate transistor having a plurality of channelformation regions, such as a double-gate transistor may be used.

A transistor with any of a variety of structures such as a planar type,a FIN-type, a Tri-Gate type, and the like can be used.

The above transistors may have the same structure or differentstructures. The size (e.g., channel length and channel width) or thelike of each transistor can be adjusted as appropriate.

In the case where all of the plurality of transistors in the imagingdevice 100 have the same structure, the transistors can be formedconcurrently in the same process.

The transistor 241 includes an electrode 243 that can function as a gateelectrode, an electrode 244 that can function as one of a sourceelectrode and a drain electrode, an electrode 245 that can function asthe other of the source electrode and the drain electrode, an insulatinglayer 117 that can function as a gate insulating layer, and asemiconductor layer 242 (see FIG. 22 ).

Note that in FIG. 21 , both of the electrode that functions as the otherof the source electrode and the drain electrode of the transistor 241and the electrode that can function as one of the source electrode andthe drain electrode of the transistor 246 are formed using the electrode245. However, one embodiment of the present invention is not limitedthereto. The electrode that functions as the other of the sourceelectrode and the drain electrode of the transistor 241 and theelectrode that can function as one of the source electrode and the drainelectrode of the transistor 246 may be formed using differentelectrodes.

The insulating layer 108 is preferably formed using an insulating filmthat has a function of preventing diffusion of impurities such asoxygen, hydrogen, water, alkali metal, and alkaline earth metal.Examples of the insulating film include silicon oxide, siliconoxynitride, silicon nitride, silicon nitride oxide, gallium oxide,hafnium oxide, yttrium oxide, aluminum oxide, aluminum oxynitride, andthe like. When the insulating film is formed using silicon nitride,gallium oxide, hafnium oxide, yttrium oxide, aluminum oxide, or thelike, impurities diffused from the photoelectric conversion element 220side can be suppressed from reaching the semiconductor layer 242. Notethat the insulating layer 108 can be formed by a sputtering method, aCVD method, an evaporation method, a thermal oxidation method, or thelike. The insulating layer 108 can be formed to have a single-layerstructure or a stacked-layer structure including any of these materials.

The insulating layer 109 can be formed using a material and a methodsimilar to those of the insulating layer 102. In the case where an oxidesemiconductor is used for the semiconductor layer 242, an insulatinglayer containing oxygen in excess of the stoichiometric composition ispreferably used for the insulating layer 108. From the insulating layercontaining oxygen at a higher proportion than oxygen in thestoichiometric composition, part of oxygen is released by heating. Theinsulating layer containing oxygen at a higher proportion than oxygen inthe stoichiometric composition is an insulating layer of which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×1018 atoms/cm3, preferably greater than or equal to3.0×1020 atoms/cm3 in TDS analysis. Note that the temperature of thefilm surface in the TDS analysis is preferably higher than or equal to100° C. and lower than or equal to 700° C., or higher than or equal to100° C. and lower than or equal to 500° C.

The insulating layer containing oxygen at a higher proportion than thestoichiometric composition can be formed by treatment for adding oxygento the insulating layer. The treatment for adding oxygen can beperformed by heat treatment under an oxygen atmosphere or performed withan ion implantation apparatus, an ion doping apparatus, or a plasmatreatment apparatus. As a gas for adding oxygen, an oxygen gas of 1602,1802, or the like, a nitrous oxide gas, an ozone gas, or the like can beused. In this specification, the treatment for adding oxygen is alsoreferred to as “oxygen doping treatment”.

Each semiconductor layer in the transistor 241, the transistor 246, thetransistor 289, and the like can be formed using a single crystalsemiconductor, a polycrystalline semiconductor, a microcrystallinesemiconductor, a nanocrystal semiconductor, a semi-amorphoussemiconductor, an amorphous semiconductor, or the like. For example,amorphous silicon or microcrystalline germanium can be used.Alternatively, a compound semiconductor such as silicon carbide, galliumarsenide, an oxide semiconductor, or a nitride semiconductor, an organicsemiconductor, or the like can be used.

In this embodiment, an example in which an oxide semiconductor is usedfor the semiconductor layer 242 is described. Furthermore, in thisembodiment, a case where the semiconductor layer 242 is a stacked layerincluding a semiconductor layer 242 a, a semiconductor layer 242 b, anda semiconductor layer 242 c is described.

Each of the semiconductor layer 242 a, the semiconductor layer 242 b,and the semiconductor layer 242 c is formed using a material containingeither In or Ga or both of them. Typical examples are an In—Ga oxide (anoxide containing In and Ga), an In—Zn oxide (an oxide containing In andZn), and an In-M-Zn oxide (an oxide containing In, an element M, and Zn:the element M is one or more kinds of metal elements selected from Al,Ti, Ga, Y, Zr, La, Ce, Nd, and Hf whose strength of bonding with oxygenis higher than that of In).

The semiconductor layer 242 a and the semiconductor layer 242 c arepreferably formed using a material containing one or more kinds of metalelements contained in the semiconductor layer 242 b. With use of such amaterial, interface states at interfaces between the semiconductor layer242 a and the semiconductor layer 242 b and between the semiconductorlayer 242 c and the semiconductor layer 242 b are less likely to begenerated. Accordingly, carriers are not likely to be scattered orcaptured at the interfaces, which results in an improvement infield-effect mobility of the transistor. Further, threshold-voltagevariation of the transistor can be reduced. Thus, a semiconductor devicehaving favorable electrical characteristics can be obtained.

Each of the thicknesses of the semiconductor layer 242 a and thesemiconductor layer 242 c is greater than or equal to 3 nm and less thanor equal to 100 nm, preferably greater than or equal to 3 nm and lessthan or equal to 50 nm. The thickness of the semiconductor layer 242 bis greater than or equal to 3 nm and less than or equal to 200 nm,preferably greater than or equal to 3 nm and less than or equal to 100nm, further preferably greater than or equal to 3 nm and less than orequal to 50 nm.

In the case where the semiconductor layer 242 b is an In-M-Zn oxide andthe semiconductor layer 242 a and the semiconductor layer 242 c are eachan In-M-Zn oxide, the semiconductor layer 242 a and the semiconductorlayer 242 c each have the atomic ratio where InM:Zn=x₁:y₁:z₁, and thesemiconductor layer 242 b has an atomic ratio where In:M:Zn=x₂:y₂:z₂,for example. In that case, the compositions of the semiconductor layer242 a, the semiconductor layer 242 c, and the semiconductor layer 242 bare determined so that y₁/x₁ is larger than y₂/x₂. It is preferable thatthe compositions of the semiconductor layer 242 a, the semiconductorlayer 242 c, and the semiconductor layer 242 b are determined so thaty₁/x₁ is 1.5 times or more as large as y₂/x₂. It is further preferablethat the compositions of the semiconductor layer 242 a, thesemiconductor layer 242 c, and the semiconductor layer 242 b aredetermined so that y₁/x₁ is twice or more as large as y₂/x₂. It is stillfurther preferable that the compositions of the semiconductor layer 242a, the semiconductor layer 242 c, and the semiconductor layer 242 b aredetermined so that y₁/x₁ is three times or more as large as y₂/x₂. Atthis time, y₁ is preferably greater than or equal to x₁ in thesemiconductor layer 242 b, in which case stable electricalcharacteristics of a transistor can be achieved. However, when y₁ isthree times or more as large as x₁, the field-effect mobility of thetransistor is reduced; accordingly, y₁ is preferably smaller than threetimes x₁. When the semiconductor layer 242 a and the semiconductor layer242 c have the above compositions, the semiconductor layer 242 a and thesemiconductor layer 242 c can each be a layer in which oxygen vacanciesare less likely to be generated than that in the semiconductor layer 242b.

In the case where the semiconductor layer 242 a and the semiconductorlayer 242 c are each an In-M-Zn oxide; the atomic percentages of In andthe element M are preferably less than 50 atoms % and more than 50 atoms%, respectively, further preferably less than 25 atomic % and more than75 atoms %, respectively, where the summation of In and M is assumed tobe 100 atomic %. In the case where the semiconductor layer 242 b is anIn-M-Zn oxide and the summation of In and M is assumed to be 100 atomic%; the atomic percentages of In and the element M are preferably morethan 25 atomic % and less than 75 atomic %, respectively, furtherpreferably more than 34 atomic % and less than 66 atomic %,respectively.

For example, an In—Ga—Zn oxide which is formed using a target having anatomic ratio of In:Ga:Zn=1:3:2, 1:3:4, 1:3:6, 1:6:4, 1:9:6, or the likeor an In—Ga oxide which is formed using a target having an atomic ratioof In:Ga=1:9 or the like can be used for each of the semiconductorlayers 242 a and 242 c containing In or Ga. Furthermore, an In—Ga—Znoxide which is formed using a target having an atomic ratio ofIn:Ga:Zn=3:1:2, 1:1:1, or 5:5:6 can be used for the semiconductor layer242 b. Note that the atomic ratio of each of the semiconductor layer 242a and the semiconductor layer 242 b may vary within a range of ±20% ofany of the above-described atomic ratios as an error.

In order to give stable electrical characteristics to the transistorincluding the semiconductor layer 242 b, it is preferable thatimpurities and oxygen vacancies in the semiconductor layer 242 b bereduced to obtain a highly purified semiconductor layer; accordingly,the semiconductor layer 242 b can be regarded as an intrinsic orsubstantially intrinsic oxide semiconductor layer. Furthermore, it ispreferable that at least the channel formation region of thesemiconductor layer 242 b be regarded as an intrinsic or substantiallyintrinsic semiconductor layer.

Note that the substantially intrinsic semiconductor layer refers to anoxide semiconductor layer in which the carrier density is lower than1×1017/cm3, lower than 1×1015/cm3, or lower than 1×1013/cm3.

Although in FIG. 21 the photoelectric conversion element 220 includesthe i-type semiconductor layer 222 positioned between the p-typesemiconductor layer 221 and the n-type semiconductor layer 223, thephotoelectric conversion element 220 may have a stacked structure asshown in FIG. 34 . In the photoelectric conversion element 220 shown inFIG. 34 , a photoelectric conversion layer 297 is sandwiched between anelectrode 268 and an electrode 298. The electrode 268, which is one ofthe electrodes of the photoelectric conversion element 220, iselectrically connected via the contact plug 119 and the like to theelectrode 245 in the transistor 241. As the photoelectric conversionlayer 297, a selenium-based semiconductor element S_(Se), described inEmbodiment 4 later, may be used, for example. For the electrodes 268 and298, the materials listed in the description of the electrode 226 may beused, for example. The electrodes 268 and 298 may be formed usingdifferent materials or using the same material. Note that it is alsopossible to form the transistors in the pixel region 251 through thesame process as the transistors 281 and 282 in the peripheral circuitregion 252, as exemplified by a transistor 281 a and a transistor 281 bshown in FIG. 37 .

Further, a p-type transistor 281, an n-type transistor 282, and thephotoelectric conversion element 220 may be formed using a semiconductorsubstrate 155, as shown in FIG. 38 . As the semiconductor substrate 155,for example, a silicon substrate can be used. An n-type semiconductorlayer 157 and a p-type semiconductor layer 156 may be used as a cathodeand an anode of the photoelectric conversion element 220, respectively,for example. It is preferable that the silicon substrate be thinned bypolishing or the like to allow light to be easily transmittedtherethrough.

[Energy Band Structure of Oxide Semiconductor]

The function and effect of the semiconductor layer 242 that is a stackedlayer including the semiconductor layer 242 a, the semiconductor layer242 b, and the semiconductor layer 242 c will be described with anenergy band structure diagram shown in FIG. 23 . FIG. 23 is the energyband structure diagram showing a portion along dashed-dotted line C1-C2in FIG. 22 . Thus, FIG. 23 illustrates the energy band structure of achannel formation region of the transistor 241.

In FIG. 23 , Ec382, Ec383 a, Ec383 b, Ec383 c, and Ec386 are theenergies of bottoms of the conduction band in the insulating layer 109,the semiconductor layer 242 a, the semiconductor layer 242 b, thesemiconductor layer 242 c, and the insulating layer 117, respectively.

Here, a difference in energy between the vacuum level and the bottom ofthe conduction band (the difference is also referred to as “electronaffinity”) corresponds to a value obtained by subtracting an energy gapfrom a difference in energy between the vacuum level and the top of thevalence band (the difference is also referred to as an ionizationpotential). Note that the energy gap can be measured using aspectroscopic ellipsometer (UT-300 manufactured by HORIBA JOBIN YVONS.A.S.). The energy difference between the vacuum level and the top ofthe valence band can be measured using an ultraviolet photoelectronspectroscopy (UPS) device (VersaProbe manufactured by ULVAC-PHI, Inc.).

An example of the energy gap and electron affinity of an In—Ga—Zn oxideformed using a sputtering method will be explained. An In—Ga—Zn oxidewhich is formed using a target having an atomic ratio of In:Ga:Zn=1:3:2has an energy gap of approximately 3.5 eV and an electron affinity ofapproximately 4.5 eV. An In—Ga—Zn oxide which is formed using a targethaving an atomic ratio of In:Ga:Zn=1:3:4 has an energy gap ofapproximately 3.4 eV and an electron affinity of approximately 4.5 eV.An In—Ga—Zn oxide which is formed using a target having an atomic ratioof In:Ga:Zn=1:3:6 has an energy gap of approximately 3.3 eV and anelectron affinity of approximately 4.5 eV. An In—Ga—Zn oxide which isformed using a target having an atomic ratio of In:Ga:Zn=1:6:2 has anenergy gap of approximately 3.9 eV and an electron affinity ofapproximately 4.3 eV. An In—Ga—Zn oxide which is formed using a targethaving an atomic ratio of In:Ga:Zn=1:6:8 has an energy gap ofapproximately 3.5 eV and an electron affinity of approximately 4.4 eV.An In—Ga—Zn oxide which is formed using a target having an atomic ratioof In:Ga:Zn=1:6:10 has an energy gap of approximately 3.5 eV and anelectron affinity of approximately 4.5 eV. An In—Ga—Zn oxide which isformed using a target having an atomic ratio of In:Ga:Zn=1:1:1 has anenergy gap of approximately 3.2 eV and an electron affinity ofapproximately 4.7 eV. An In—Ga—Zn oxide which is formed using a targethaving an atomic ratio of In:Ga:Zn=3:1:2 has an energy gap ofapproximately 2.8 eV and an electron affinity of approximately 5.0 eV.Note that values obtained as the energy gap and electron affinity of anIn—Ga—Zn oxide formed using a sputtering method may change depending onthe film formation conditions of the sputtering method.

Since the insulating layer 109 and the insulating layer 117 areinsulators, Ec382 and Ec386 are closer to the vacuum level (have asmaller electron affinity) than Ec383 a, Ec383 b, and Ec383 c.

Further, Ec383 a is closer to the vacuum level than Ec383 b.Specifically, Ec383 a is preferably located closer to the vacuum levelthan Ec383 b by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4eV or less.

Further, Ec383 c is closer to the vacuum level than Ec383 b.Specifically, Ec383 c is preferably located closer to the vacuum levelthan Ec383 b by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4eV or less.

In the vicinity of an interface between the semiconductor layer 242 aand the semiconductor layer 242 b and the vicinity of an interfacebetween the semiconductor layer 242 b and the semiconductor layer 242 c,mixed regions are formed; thus, the energy of the bottom of theconduction band continuously changes. In other words, no state or fewstates exist at these interfaces.

Accordingly, electrons transfer mainly through the semiconductor layer242 b in the stacked-layer structure having the above energy bandstructure. Therefore, even when an interface state exists at aninterface between the semiconductor layer 242 a and the insulating layer109 or an interface between the semiconductor layer 242 c and theinsulating layer 117, the interface state hardly influences the transferof the electrons. In addition, the interface state does not exist orhardly exists at the interface between the semiconductor layer 242 a andthe semiconductor layer 242 b and at the interface between thesemiconductor layer 242 c and the semiconductor layer 242 b; thus,transfer of electrons are not prohibited in the region. Accordingly,high field-effect mobility can be obtained in the transistor 241 havingthe above stacked-layer structure of the oxide semiconductor layers.

Note that although trap states 390 due to impurities or defects might beformed in the vicinity of the interface between the semiconductor layer242 a and the insulating layer 109 and in the vicinity of the interfacebetween the semiconductor layer 242 c and the insulating layer 117 asshown in FIG. 23 , the semiconductor layer 242 b can be separated fromthe trap states owing to the existence of the semiconductor layer 242 aand the semiconductor layer 242 c.

In particular, in the transistor 241 described in this embodiment, anupper surface and a side surface of the semiconductor layer 242 b are incontact with the semiconductor layer 242 c, and a bottom surface of thesemiconductor layer 242 b is in contact with the semiconductor layer 242a. In this manner, the semiconductor layer 242 b is surrounded by thesemiconductor layer 242 a and the semiconductor layer 242 c, whereby theinfluence of the trap state can be further reduced.

However, in the case where an energy difference between Ec383 a or Ec383c and Ec383 b is small, electrons in the semiconductor layer 242 b mightreach the trap states by passing over the energy gap. The electrons aretrapped by the trap states, which generates a negative fixed charge atthe interface with the insulating layer, causing the threshold voltageof the transistor to be shifted in the positive direction.

Therefore, each of the energy differences between Ec383 a and Ec383 band between Ec383 c and Ec383 b is preferably set to be larger than orequal to 0.1 eV, more preferably larger than or equal to 0.15 eV, inwhich case a change in the threshold voltage of the transistor can bereduced and the transistor can have favorable electricalcharacteristics.

Each of the band gaps of the semiconductor layer 242 a and thesemiconductor layer 242 c is preferably larger than that of thesemiconductor layer 242 b.

With one embodiment of the present invention, a transistor with a smallvariation in electrical characteristics can be provided. Accordingly, asemiconductor device with a small variation in electricalcharacteristics can be provided. With one embodiment of the presentinvention, a transistor with high reliability can be provided.Accordingly, a semiconductor device with high reliability can beprovided.

An oxide semiconductor has a band gap of 2 eV or more; therefore, atransistor including an oxide semiconductor in a semiconductor layer inwhich a channel is formed has an extremely small off-state current.Specifically, the off-state current per micrometer of channel width atroom temperature can be lower than 1×10⁻²⁰ A, preferably lower than1×10⁻²² A, more preferably lower than 1×10⁻²⁴ A. That is, the on/offratio of the transistor can be greater than or equal to 20 digits andless than or equal to 150 digits.

In a transistor that transfers the charge obtained through photoelectricconversion of a photodiode to a charge accumulation region, such as thetransistor 121 and the transistor 125 in FIG. 5 , the channel width isincreased to improve the transfer efficiency in some cases. In suchcases, the use of an oxide semiconductor for the transistors can achieveextremely small off-state current, and leakage of charge in a chargeaccumulation region, e.g., leakage of the charge accumulated in thenodes FD[n] and FD[n+1] in FIG. 5 , can be suppressed. Thus, imagingtime can be shortened. Furthermore, an imaging device with highreliability can be provided.

With one embodiment of the present invention, a transistor with smallpower consumption can be provided. Accordingly, a semiconductor devicewith small power consumption can be provided.

Furthermore, since an oxide semiconductor has a wide bandgap, asemiconductor device including an oxide semiconductor can be used in awide range of ambient temperature. According to one embodiment of thepresent invention, a semiconductor device which can operate in a widetemperature range can be provided.

Note that the above-described three-layer structure is an example. Forexample, a two-layer structure without either one of the semiconductorlayer 242 a and the semiconductor layer 242 c may be employed.

[Oxide Semiconductor]

An oxide semiconductor applicable to the semiconductor layer 242 will bedescribed in detail below.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.The term “substantially parallel” indicates that the angle formedbetween two straight lines is greater than or equal to −30° and lessthan or equal to 30°. In addition, the term “perpendicular” indicatesthat the angle formed between two straight lines is greater than orequal to 80° and less than or equal to 100°, and accordingly alsoincludes the case where the angle is greater than or equal to 85° andless than or equal to 95°. The term “substantially perpendicular”indicates that the angle formed between two straight lines is greaterthan or equal to 60° and less than or equal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.In the case of using an oxide semiconductor for the semiconductor layer,a c-axis aligned crystalline oxide semiconductor (CAAC-OS), apolycrystalline oxide semiconductor, a microcrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous oxide semiconductor, or the like can be used.

First, a CAAC-OS film will be described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image(also referred to as a high-resolution TEM image) of a bright-fieldimage and a diffraction pattern of the CAAC-OS film is observed.However, in the high-resolution TEM image, a boundary between crystalparts, that is, a grain boundary is not clearly observed. Thus, in theCAAC-OS film, a reduction in electron mobility due to the grain boundaryis less likely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface, metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a form reflecting unevenness of asurface over which the CAAC-OS film is formed (hereinafter, a surfaceover which the CAAC-OS film is formed is referred to as a formationsurface) or a top surface of the CAAC-OS film, and is arranged parallelto the formation surface or the top surface of the CAAC-OS film.

According to the high-resolution plan-view TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface, metal atoms are arranged in a triangular or hexagonalconfiguration in the crystal parts. However, there is no regularity ofarrangement of metal atoms between different crystal parts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor which includesthe oxide semiconductor film having high impurity concentration and ahigh density of defect states has unstable electrical characteristics insome cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor film will be described.

A microcrystalline oxide semiconductor film has a region where a crystalpart is observed in a high resolution TEM image and a region where acrystal part is not clearly observed in a high resolution TEM image. Inmost cases, a crystal part in the microcrystalline oxide semiconductoris greater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In a high resolution TEMimage of the nc-OS film, a grain boundary cannot be found clearly in thenc-OS film sometimes for example.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Note that there isno regularity of crystal orientation between different crystal parts inthe nc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor film depending on an analysis method.For example, when the nc-OS film is subjected to structural analysis byan out-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. A diffraction pattern like a halo patternappears in a selected-area electron diffraction pattern of the nc-OSfilm obtained by using an electron beam having a probe diameter (e.g.,larger than or equal to 50 nm) larger than the diameter of a crystalpart. Meanwhile, spots are shown in a nanobeam electron diffractionpattern of the nc-OS film obtained by using an electron beam having aprobe diameter close to, or smaller than the diameter of a crystal part.Further, in a nanobeam electron diffraction pattern of the nc-OS film,regions with high luminance in a circular (ring) pattern are shown insome cases. Also in a nanobeam electron diffraction pattern of the nc-OSfilm, a plurality of spots is shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. Note that there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Next, an amorphous oxide semiconductor film will be described.

The amorphous oxide semiconductor film has disordered atomic arrangementand no crystal part. For example, the amorphous oxide semiconductor filmdoes not have a specific state as in quartz.

In the high-resolution TEM image of the amorphous oxide semiconductorfilm, crystal parts cannot be found.

When the amorphous oxide semiconductor film is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is shown in anelectron diffraction pattern of the amorphous oxide semiconductor film.Further, a halo pattern is shown but a spot is not shown in a nanobeamelectron diffraction pattern of the amorphous oxide semiconductor film.

Note that an oxide semiconductor film may have a structure havingphysical properties between the nc-OS film and the amorphous oxidesemiconductor film. The oxide semiconductor film having such a structureis specifically referred to as an amorphous-like oxide semiconductor(a-like OS) film.

In a high-resolution TEM image of the a-like OS film, a void may beseen. Furthermore, in the high-resolution TEM image, there are a regionwhere a crystal part is clearly observed and a region where a crystalpart is not observed. In the a-like OS film, crystallization by a slightamount of electron beam used for TEM observation occurs and growth ofthe crystal part is found sometimes. In contrast, crystallization by aslight amount of electron beam used for TEM observation is less observedin the nc-OS film having good quality.

Note that the crystal part size in the a-like OS film and the nc-OS filmcan be measured using high-resolution TEM images. For example, anInGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers areincluded between In—O layers. A unit cell of the InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are layered in the c-axis direction. Accordingly, thespacing between these adjacent layers is equivalent to the latticespacing on the (009) plane (also referred to as d value). The value iscalculated to 0.29 nm from crystal structure analysis. Thus, focusing onlattice fringes in the high-resolution TEM image, each of latticefringes in which the lattice spacing therebetween is greater than orequal to 0.28 nm and less than or equal to 0.30 nm corresponds to thea-b plane of the InGaZnO₄ crystal.

The density of an oxide semiconductor film might vary depending on itsstructure. For example, if the composition of an oxide semiconductorfilm is determined, the structure of the oxide semiconductor film can beestimated from a comparison between the density of the oxidesemiconductor film and the density of a single crystal oxidesemiconductor film having the same composition as the oxidesemiconductor film. For example, the density of the a-like OS film ishigher than or equal to 78.6% and lower than 92.3% of the density of thesingle crystal oxide semiconductor having the same composition. Forexample, the density of each of the nc-OS film and the CAAC-OS film ishigher than or equal to 92.3% and lower than 100% of the density of thesingle crystal oxide semiconductor having the same composition. Notethat it is difficult to deposit an oxide semiconductor film whosedensity is lower than 78% of the density of the single crystal oxidesemiconductor film.

Specific examples of the above description will be given below. Forexample, for an oxide semiconductor film with an atomic ratio ofIn:Ga:Zn=1:1:1, the density of single-crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Thus, for example, forthe oxide semiconductor film with an atomic ratio of In:Ga:Zn=1:1:1, thedensity of an a-like OS film is higher than or equal to 5.0 g/cm³ andlower than 5.9 g/cm³. In addition, for example, for the oxidesemiconductor film with an atomic ratio of In:Ga:Zn=1:1:1, the densityof an nc-OS film or a CAAC-OS film is higher than or equal to 5.9 g/cm³and lower than 6.3 g/cm³.

Note that single crystals with the same composition do not exist in somecases. In such a case, by combining single crystals with differentcompositions at a given proportion, it is possible to calculate densitythat corresponds to the density of a single crystal with a desiredcomposition. The density of the single crystal with a desiredcomposition may be calculated using weighted average with respect to thecombination ratio of the single crystals with different compositions.Note that it is preferable to combine as few kinds of single crystals aspossible for density calculation.

Note that an oxide semiconductor film may be a stacked film includingtwo or more films of an amorphous oxide semiconductor film, an a-like OSfilm, a microcrystalline oxide semiconductor film, and a CAAC-OS film,for example.

As an example of an oxide semiconductor that can be used for thesemiconductor layer 242 a, the semiconductor layer 242 b, and thesemiconductor layer 242 c, an oxide containing indium can be given. Anoxide can have a high carrier mobility (electron mobility) by containingindium, for example. An oxide semiconductor preferably contains anelement M. The element M is preferably aluminum, gallium, yttrium, tin,or the like. Other elements which can be used as the element M areboron, silicon, titanium, iron, nickel, germanium, yttrium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like. Note that two or more of the above elements maybe used in combination as the element M. The element M is an elementhaving a high bonding energy with oxygen, for example. The element M isan element that can increase the energy gap of the oxide, for example.Further, the oxide semiconductor preferably contains zinc. When theoxide contains zinc, the oxide is easily to be crystallized, forexample.

Note that the oxide semiconductor is not limited to the oxide containingindium. The oxide semiconductor may be, for example, zinc tin oxide,gallium tin oxide, or gallium oxide.

For the oxide semiconductor, an oxide with a wide energy gap is used.For example, the energy gap of the oxide semiconductor is greater thanor equal to 2.5 eV and less than or equal to 4.2 eV, preferably greaterthan or equal to 2.8 eV and less than or equal to 3.8 eV, morepreferably greater than or equal to 3 eV and less than or equal to 3.5eV.

Influence of impurities in the oxide semiconductor will be describedbelow. In order to obtain stable electrical characteristics of atransistor, it is effective to reduce the concentration of impurities inthe oxide semiconductor to have lower carrier density so that the oxidesemiconductor is highly purified. The carrier density of the oxidesemiconductor is set to be lower than 1×10¹⁷/cm³, lower than 1×10¹⁵/cm³,or lower than 1×10¹³/cm³. In order to reduce the concentration ofimpurities in the oxide semiconductor, the concentration of impuritiesin a film which is adjacent to the oxide semiconductor is preferablyreduced.

For example, silicon in the oxide semiconductor might serve as a carriertrap or a carrier generation source. The silicon concentration in theoxide semiconductor measured by secondary ion mass spectrometry (SIMS)is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³,more preferably lower than 2×10¹⁸ atoms/cm³.

Furthermore, when hydrogen is contained in the oxide semiconductor, thecarrier density is increased in some cases. Thus, the concentration ofhydrogen in the oxide semiconductor, which is measured by SIMS, can beset to lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than orequal to 5×10¹⁹ atoms/cm³, more preferably lower than or equal to 1×10¹⁹atoms/cm³, still more preferably lower than or equal to 5×10¹⁸atoms/cm³. When nitrogen is contained in the oxide semiconductor, thecarrier density is increased in some cases. The concentration ofnitrogen in the oxide semiconductor measured by SIMS is set to be lowerthan 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, more preferably lower than or equal to 1×10¹⁸ atoms/cm³,still more preferably lower than or equal to 5×10¹⁷ atoms/cm³.

In order to reduce the hydrogen concentration in the oxidesemiconductor, the hydrogen concentrations in the insulating layer 109and the insulating layer 117 that are in contact with the semiconductorlayer 242 are preferably reduced. The hydrogen concentration in theinsulating layer 109 and the insulating layer 117 measured by SIMS islower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equalto 5×10¹⁹ atoms/cm³, more preferably lower than or equal to 1×10¹⁹atoms/cm³, still more preferably lower than or equal to 5×10¹⁸atoms/cm³. In order to reduce the nitrogen concentration in the oxidesemiconductor, the nitrogen concentrations in the insulating layer 109and the insulating layer 117 are preferably reduced. The nitrogenconcentration in the insulating layer 109 and the insulating layer 117measured by SIMS is lower than 5×10¹⁹ atoms/cm³, preferably lower thanor equal to 5×10¹⁸ atoms/cm³, more preferably lower than or equal to1×10¹⁸ atoms/cm³, still more preferably lower than or equal to 5×10¹⁷atoms/cm³.

In this embodiment, first, the semiconductor layer 242 a is formed overthe insulating layer 109, and the semiconductor layer 242 b is formedover the semiconductor layer 242 a.

A sputtering method is preferably used for formation of the oxidesemiconductor layers. As a sputtering method, an RF sputtering method, aDC sputtering method, an AC sputtering method, or the like can be used.A DC sputtering method or an AC sputtering method can achieve uniformdeposition as compared to an RF sputtering method.

In this embodiment, as the semiconductor layer 242 a, 20-nm-thickIn—Ga—Zn oxide is deposited by a sputtering method using an In—Ga—Znoxide target (In:Ga:Zn=1:3:2). Note that the constituent elements andcompositions applicable to the semiconductor layer 242 a are not limitedthereto.

The oxygen doping treatment may be performed after the formation of thesemiconductor layer 242 a.

Next, the semiconductor layer 242 b is formed over the semiconductorlayer 242 a. In this embodiment, as the semiconductor layer 242 b,30-nm-thick In—Ga—Zn oxide is deposited by a sputtering method using anIn—Ga—Zn oxide target (In:Ga:Zn=1:1:1). Note that the constituentelements and compositions applicable to the semiconductor layer 242 bare not limited thereto.

The oxygen doping treatment may be performed after the formation of thesemiconductor layer 242 b.

Next, heat treatment may be performed to further reduce the impuritiessuch as moisture or hydrogen contained in the semiconductor layer 242 aand the semiconductor layer 242 b, so that the semiconductor layer 242 aand the semiconductor layer 242 b are highly purified.

For example, the semiconductor layer 242 a and the semiconductor layer242 b are subjected to heat treatment in a reduced-pressure atmosphere,an inert gas atmosphere of nitrogen, a rare gas, or the like, anoxidation atmosphere, or an ultra dry air atmosphere (the moistureamount is 20 ppm (−55° C. by conversion into a dew point) or less,preferably 1 ppm or less, more preferably 10 ppb or less, in the casewhere the measurement is performed by a dew point meter in a cavity ringdown laser spectroscopy (CRDS) system). Note that the oxidationatmosphere refers to an atmosphere including an oxidation gas such asoxygen, ozone, or nitrogen oxide at 10 ppm or higher. The inert gasatmosphere refers to an atmosphere including the oxidation gas at lowerthan 10 ppm and is filled with nitrogen or a rare gas.

By heat treatment, oxygen included in the insulating layer 109 can bediffused into the semiconductor layer 242 a and the semiconductor layer242 b, concurrently with the release of impurities, so that oxygenvacancies in the semiconductor layer 242 a and the semiconductor layer242 b can be reduced. Note that the heat treatment may be performed insuch a manner that heat treatment is performed in an inert gasatmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more, 1% or more, or10% or more. The heat treatment may be performed at any time after thesemiconductor layer 242 b is formed. For example, the heat treatment maybe performed after the semiconductor layer 242 b is selectively etched.

The heat treatment can be performed at a temperature higher than orequal to 250° C. and lower than or equal to 650° C., preferably higherthan or equal to 300° C. and lower than or equal to 500° C. Thetreatment time is shorter than or equal to 24 hours. Heat treatment forover 24 hours is not preferable because the productivity is reduced.

Next, a resist mask is formed over the semiconductor layer 242 b, andwith use of the resist mask, part of the semiconductor layer 242 a andpart of the semiconductor layer 242 b are etched selectively. At thistime, the insulating layer 109 might be partly etched, thereby having aprojection.

Either of a dry etching method or a wet etching method may be used foretching of the semiconductor layer 242 a and the semiconductor layer 242b, or both of them may be used. After the etching, the resist mask isremoved.

In the transistor 241, the electrode 244 and the electrode 245 that arein contact with part of the semiconductor layer 242 b are provided overthe semiconductor layer 242 b. The electrode 244 and the electrode 245(including another electrode or wiring that is formed in the same layeras these electrodes) can be formed using a material and a method similarto those of the wiring 226.

In addition, the transistor 241 includes the semiconductor layer 242 cover the semiconductor layer 242 b, the electrode 244, and the electrode245. The semiconductor layer 242 c is partly in contact with each of thesemiconductor layer 242 b, the electrode 244, and the electrode 245.

In this embodiment, the semiconductor layer 242 c is formed by asputtering method using an In—Ga—Zn oxide target (In:Ga:Zn=1:3:2). Notethat the constituent elements and compositions applicable to thesemiconductor layer 242 c are not limited thereto. For example, oxidegallium may be used for the semiconductor layer 242 c. Furthermore,oxygen doping treatment may be performed on the semiconductor layer 242c.

Furthermore, in the transistor 241, the insulating layer 117 is providedover the semiconductor layer 242 c. The insulating layer 117 canfunction as a gate insulating layer. The insulating layer 117 can beformed using a material and a method similar to those of the insulatinglayer 102. The oxygen doping treatment may be performed on theinsulating layer 117.

After the semiconductor layer 242 c and the insulating layer 117 areformed, a mask is formed over the insulating layer 117, and parts of thesemiconductor layer 242 c and the insulating layer 117 are selectivelyetched, so that the semiconductor layer 242 c and the insulating layer117 may be formed into island shapes.

Moreover in the transistor 241, the electrode 243 is provided over theinsulating layer 117. The electrode 243 (including another electrode orwiring that is formed in the same layer as this electrode) can be formedusing a material and a method similar to those of the wiring 226.

In this embodiment, an example in which the electrode 243 has astacked-layer structure including an electrode 243 a and an electrode243 b is shown. For example, the electrode 243 a is formed usingtantalum nitride, and the electrode 243 b is formed using copper. Theelectrode 243 a functions as a barrier layer to prevent copperdiffusion. Thus, a semiconductor device with high reliability can beobtained.

Moreover, the transistor 241 includes an insulating layer 118 coveringthe electrode 243. The insulating layer 118 can be formed using amaterial and a method similar to those of the insulating layer 102. Theoxygen doping treatment may be performed on the insulating layer 117.Furthermore, a surface of the insulating layer 118 may be subjected toCMP treatment. The CMP treatment can reduce unevenness of the surface,whereby coverage with an insulating layer or a conductive layer to beformed later can be increased.

In addition, an insulating layer 113 is formed over the insulating layer118. The insulating layer 113 can be formed using a material and amethod similar to those of the insulating layer 105. A surface of theinsulating layer 113 may be subjected to CMP treatment. The CMPtreatment can reduce unevenness of the surface, whereby coverage with aninsulating layer or a conductive layer to be formed later can beincreased. In addition, an opening is formed in parts of the insulatinglayer 113 and the insulating layer 118. Contact plugs 114 are formed inthe openings.

Over the insulating layer 113, a wiring 261, a wiring 265, and a wiring267 (including another electrode or wiring formed in the same layer asthe wirings) are formed. The wiring 267 is electrically connected to anelectrode 249 via the contact plug 114 in the opening formed through theinsulating layer 113 and the insulating layer 118. The wiring 265 iselectrically connected to the electrode 244 via the contact plug 114 inthe opening formed in the insulating layer 113 and the insulating layer118.

The imaging device 100 includes an insulating layer 115 to cover thewiring 261, the wiring 265, and the wiring 267 (including anotherelectrode or wiring formed in the same layer as the wirings). Theinsulating layer 115 can be formed using a material and a method similarto those of the insulating layer 105. A surface of the insulating layer115 may be subjected to CMP treatment. The CMP treatment can reduceunevenness of the surface, and coverage whereby an insulating layer or aconductive layer to be formed later can be increased. In addition, anopening is formed in part of the insulating layer 115. A contact plug114 is formed in the opening.

Further, a wiring 263 and a wiring 266 (including another electrode orwiring that is formed in the same layer as these wirings) are formedover the insulating layer 115.

Each of the wiring 263 and the wiring 266 (including another electrodeor wiring formed in the same layer as the wirings) can be electricallyconnected to another wiring or another electrode via an opening and acontact plug formed through the insulating layer.

Further, an insulating layer 116 is provided to cover the wiring 263 andthe wiring 266. The insulating layer 116 can be formed using a materialand a method similar to those of the insulating layer 105. A surface ofthe insulating layer 116 may be subjected to CMP treatment.

The transistor 241 illustrated in FIG. 21 corresponds to the transistor121, for example. When a transistor included in a pixel is provided overthe photoelectric conversion element 220, an area occupied by thephotoelectric conversion element 220 can be increased in a plan view.Accordingly, the light sensitivity of the imaging device 100 can beimproved. Moreover, it is possible to provide the imaging device 100whose light sensitivity is less likely to decrease even when theresolution is increased.

FIG. 24A is an enlarged cross-sectional view of the transistor 281illustrated in FIG. 21 as an example of a transistor included in aperipheral circuit. FIG. 24B is an enlarged cross-sectional view of thetransistor 282 illustrated in FIG. 21 . In this embodiment, thetransistor 281 is a p-channel transistor, and the transistor 282 is ann-channel transistor, for example.

The transistor 281 includes an i-type semiconductor layer 283 in which achannel is formed, p-type semiconductor layers 285, an insulating layer286, an electrode 287, and sidewalls 288. At a region overlapping withthe sidewall 288 in the i-type semiconductor layer 283,low-concentration p-type impurity regions 284 are provided.

The i-type semiconductor layer 283 included in the transistor 281 can beformed in a step of forming the i-type semiconductor layer 222 in thephotoelectric conversion element 220 at the same time. The p-typesemiconductor layer 285 included in the transistor 281 can be formed ina step of forming the p-type semiconductor layer 221 in thephotoelectric conversion element 220 at the same time.

The insulating layer 286 can function as a gate insulating layer. Theelectrode 287 can function as a gate electrode. The low-concentrationp-type impurity regions 284 can be formed in such a manner that animpurity element is added with the use of the electrode 287 as a maskafter formation of the electrode 287 and before the formation of thesidewalls 288. In other words, the low-concentration p-type impurityregions 284 can be formed in a self-aligned manner. Thelow-concentration p-type impurity regions 284 have the same conductivitytype as that of the p-type semiconductor layer 285, and lowerconcentration of impurities imparting conductivity type than the p-typesemiconductor layer 285.

The transistor 282 has a structure similar to that of the transistor281; however, there is a difference in that low-concentration n-typeimpurity regions 294 and an n-type semiconductor layer 295 are provided,instead of the low-concentration p-type impurity regions 284 and thep-type semiconductor layer 285.

The n-type semiconductor layers 295 included in the transistor 282 canbe formed in a step of forming the n-type semiconductor layers 223 inthe photoelectric conversion element 220 at the same time. As in thecase of the transistor 281, the low-concentration n-type impurityregions 294 can be formed in a self-aligned manner. Thelow-concentration n-type impurity regions 294 have the same conductivitytype as that of the n-type semiconductor layers 295 and lowerconcentration of impurities imparting the conductivity type than then-type semiconductor layers 295.

Although the variety of films such as the metal film, the semiconductorfilm, the inorganic insulating film which are disclosed in thisspecification and the like can be formed by a sputtering method or aplasma chemical vapor deposition (CVD) method, such films may be formedby another method, for example, a thermal CVD method. A metal organicchemical vapor deposition (MOCVD) method or an atomic layer deposition(ALD) method, for example, may be employed as a thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied at a time to the chamber,in which the pressure is set to an atmospheric pressure or a reducedpressure, and react with each other in the vicinity of the substrate orover the substrate.

Deposition by an ALD method may be performed in such a manner thatsource gases for reaction are sequentially introduced into the chamber,in which the pressure is set to an atmospheric pressure or a reducedpressure, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a first layer;then the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeatedplural times until a desired thickness is obtained, whereby a thin filmwith excellent step coverage can be formed. The thickness of the thinfilm can be adjusted by the number of repetition times of the sequenceof the gas introduction; therefore, an ALD method makes it possible toaccurately adjust a thickness and thus is suitable for manufacturing aminute field effect transistor (FET).

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film which have been disclosed in theembodiment can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, for forming an In—Ga—Zn—O film,trimethylindium, trimethylgallium, and dimethylzinc are used. Thechemical formula of trimethylindium is In(CH₃)₃. The chemical formula oftrimethylindium is Ga(CH₃)₃. The chemical formula of dimethylzinc isZn(CH₃)₂. Without limitation to the above combination, triethylgallium(chemical formula: Ga(C₂H₅)₃) can be used instead of trimethylgallium,and diethylzinc (chemical formula: Zn(C₂H₅)₂) can be used instead ofdimethylzinc.

For example, in the case where a hafnium oxide film is formed with adeposition apparatus employing ALD, two kinds of gases, i.e., ozone (O₃)as an oxidizer and a source material gas which is obtained by vaporizingliquid containing a solvent and a hafnium precursor compound (hafniumalkoxide or hafnium amide such as tetrakis(dimethylamide)hafnium(TDMAH)) are used. The chemical formula oftetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of anothermaterial liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed by adeposition apparatus using an ALD method, two kinds of gases, e.g., H₂Oas an oxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. The chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by adeposition apparatus using an ALD method, hexachlorodisilane is adsorbedon a surface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced a plurality of times to form an initial tungstenfilm, and then a WF₆ gas and an H₂ gas are concurrently introduced, sothat a tungsten film is formed. Note that an SiH₄ gas may be usedinstead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas) are sequentially introduced a plurality oftimes to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas) aresequentially introduced plural times to form a GaO layer, and then aZn(CH₃)₂ gas and an O₃ gas) are sequentially introduced plural times toform a ZnO layer. Note that the order of these layers is not limited tothis example. A mixed compound layer such as an In—Ga—O layer, anIn—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing these gases.Note that although an H₂O gas which is obtained by bubbling water withan inert gas such as Ar may be used instead of an O₃ gas), it ispreferable to use an O₃ gas), which does not contain H. Further, insteadof an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃gas, a Ga(C₂H₅)₃ gas may be used. Furthermore, a Zn(CH₃)₂ gas may beused.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Embodiment 4

The peripheral circuit and the pixel circuit can be provided with, asappropriate, a logic circuit such as an OR circuit, an AND circuit, aNAND circuit, and a NOR circuit, an inverter circuit, a buffer circuit,a shift register circuit, a flip-flop circuit, an encoder circuit, adecoder circuit, an amplifier circuit, an analog switch circuit, anintegrator circuit, a differentiation circuit, a memory element, and thelike.

In this embodiment, an example of a CMOS circuit that can be used forthe peripheral circuit and the pixel circuit, or the like will bedescribed with reference to FIGS. 25A to 25E. In the circuit diagrams inFIGS. 25A to 25E, the indication of “OS” is given beside a transistor inorder to clearly demonstrate that the transistor includes an oxidesemiconductor.

The CMOS circuit shown in FIG. 25A has a configuration of what is calledan inverter circuit in which the p-channel transistor 281 and then-channel transistor 282 are connected to each other in series and inwhich gates of the transistors are connected to each other.

The CMOS circuit shown in FIG. 25B has a configuration of what is calledan analog switch circuit in which the p-channel transistor 281 and then-channel transistor 282 are connected to each other in parallel.

The circuit shown in FIG. 25C has a configuration of what is called amemory element in which one of a source and a drain of the n-channeltransistor 289 is connected to a gate of the p-channel transistor andone electrode of a capacitor 257. The circuit shown in FIG. 25D has aconfiguration of what is called a memory element in which one of asource and a drain of the n-channel transistor 289 is connected to oneelectrode of the capacitor 257.

In each of the circuits shown in FIGS. 25C and 25D, charge injected fromthe other of the source and the drain of the transistor 289 can bestored at a node 256. The transistor 289 is a transistor including anoxide semiconductor, which enables charge to be stored at the node 256for a long period. The transistor 281 may also be a transistor includingan oxide semiconductor in a semiconductor layer in which a channel isformed.

The circuit shown in FIG. 25E has a configuration example of an opticalsensor. In FIG. 25E, one of a source and a drain of a transistor 292using an oxide semiconductor for a semiconductor layer in which achannel is formed is electrically connected to a photodiode 291, and theother of the source and the drain of the transistor 292 is electricallyconnected to a gate of a transistor 293 via a node 254. The transistor292 using an oxide semiconductor for a semiconductor layer in which achannel is formed has the extremely small amount of off-state current;thus, the potential of the node 254 that is determined in accordancewith the amount of received light hardly changes. Thus, an imagingdevice which is less likely to be affected by noise can be provided.Further, an imaging device with high linearity can be provided.

The photodiode 291 in the circuit diagram of FIG. 25E may be a sensorSis.

An element which is capable of converting a given physical amount intothe amount of current Is flowing in the element is preferable as thesensor Sis. Alternatively, an element which is capable of converting agiven physical amount into another physical amount and then convertingit into the amount of current flowing in the element is preferable.

For the sensor S_(IS), a variety of sensors can be used. For example,the sensor Sis can be a temperature sensor, an optical sensor, a gassensor, a flame sensor, a smoke sensor, a humidity sensor, a pressuresensor, a flow sensor, a vibration sensor, a voice sensor, a magneticsensor, a radiation sensor, a smell sensor, a pollen sensor, anacceleration sensor, an inclination sensor, a gyro sensor, a directionsensor, or a power sensor.

For example, when an optical sensor is used as the sensor S_(IS), theabove-described photodiode or a phototransistor can be used.

When a gas sensor is used as the sensor Sis, a semiconductor gas sensorwhich detects change in resistance of a metal oxide semiconductor suchas tin oxide due to its contact with a gas, a catalytic combustion typegas sensor, or a solid electrolyte-type gas sensor can be used.

Further, a circuit diagram in which the photodiode 291 in the opticalsensor shown in FIG. 25E is a photoelectric conversion element thatincludes a selenium-based semiconductor element S_(Se) is shown in FIG.26A as an example.

A photoelectric conversion element including the selenium-basedsemiconductor element S_(Se) is an element which is capable ofconducting photoelectric conversion utilizing a phenomenon calledavalanche multiplication, in which a plurality of electrons can be takenfrom one incident photon by application of voltage. Therefore, in theoptical sensor using the photoelectric conversion element including theselenium-based semiconductor element S_(Se), the gain of electrons tothe amount of incident light can be large; therefore, a highly sensitivesensor can be obtained.

For the selenium-based semiconductor element S_(Se), a selenium-basedsemiconductor including an amorphous structure or a selenium-basedsemiconductor including a crystalline structure can be used. Forexample, the selenium-based semiconductor including a crystallinestructure may be obtained in such a manner that a selenium-basedsemiconductor including an amorphous structure is deposited andsubjected to heat treatment. Note that it is preferable that the crystalgrain diameter of the selenium-based semiconductor including acrystalline structure be smaller than a pixel pitch because variation incharacteristics of the pixels is reduced and the image quality of animage to be obtained becomes uniform.

A selenium-based semiconductor including a crystalline structure amongthe selenium-based semiconductors S_(Se) has a characteristic of havinga light absorption coefficient in a wide wavelength range. Therefore,the selenium-based semiconductor including a crystalline structure canbe used as an imaging element for light in a wide wavelength range, suchas visible light, ultraviolet light, X-rays, and gamma rays, and can beused as what is called a direct conversion element, which is capable ofdirectly converting light in a short wavelength range, such as X-raysand gamma rays, into charge.

FIG. 26B is a cross-sectional schematic view corresponding to part ofthe circuit configuration of FIG. 26A. FIG. 26B illustrates transistorsM1, electrodes E_(Pix) connected to the transistors M1, theselenium-based semiconductor elements S_(Se), an electrode E_(VPD), anda substrate Sub.

Light is emitted from the side where the electrode E_(VPD) and thesubstrate Sub are formed toward the selenium-based semiconductorelements S_(Se). Therefore, the electrode E_(VPD) and the substrate Subpreferably transmit light. Indium tin oxide can be used for theelectrode E_(VPD), and a glass substrate can be used as the substrateSub.

The selenium-based semiconductor elements S_(Se) and the electrodesE_(VPD) stacked over the selenium-based semiconductor elements S_(Se)can be used without being processed in their shapes in accordance witheach pixel. A step for processing the shape can be omitted, leading to areduction in the manufacturing cost and improvement in the manufacturingyield.

For example, a chalcopyrite-based semiconductor can be used for theselenium-based semiconductor element S_(Se). Specifically,CuIn_(1-x)Ga_(x)Se₂ (0≤x≤1, abbreviated to CIGS) can be used, forexample. CIGS can be formed by an evaporation method, a sputteringmethod, or the like.

The selenium-based semiconductor element S_(Se) formed using achalcopyrite-based semiconductor can perform avalanche multiplication bybeing applied with a voltage of several volts (from 5 V to 20 V). Byapplication of voltage to the selenium-based semiconductor elementS_(Se), the movement of signal charge generated owing to lightirradiation can have high linearity. Note that when the thickness of theselenium-based semiconductor element S_(Se) is smaller than or equal to1 μm, the application voltage can be made smaller.

Note that in the case where the thickness of the selenium-basedsemiconductor element S_(Se) is small, dark current flows at the time ofapplication of voltage; however, providing a layer for inhibiting darkcurrent from flowing in the CIGS that is a chalcopyrite-basedsemiconductor (hole-injection barrier layer) can prevent the darkcurrent from flowing. An oxide semiconductor such as gallium oxide canbe used for the hole-injection barrier layer. The thickness of thehole-injection barrier layer is preferably smaller than that of theselenium-based semiconductor element S_(Se).

FIG. 26C is a schematic cross-sectional view different from that of FIG.26B. FIG. 26C shows hole-injection barrier layers Eos together with thetransistors M1, the electrodes E_(Pix) connected to the transistors M1,the selenium-based semiconductor elements S_(Se), the electrode E_(VPD),and the substrate Sub.

As described above, use of the selenium-based semiconductor elementS_(Se) as a sensor can reduce the manufacturing cost and characteristicvariation among pixels and improves the manufacturing yield; as aresult, a highly sensitive sensor can be obtained.

For the peripheral circuit, a circuit in which a shift register circuit1800 and a buffer circuit 1900 are combined, shown in FIG. 27A, may beprovided. Alternatively, for the peripheral circuit, a circuit in whicha shift register circuit 1810, a buffer circuit 1910, and an analogswitch circuit 2100 are combined, as shown in FIG. 27B, may be provided.Vertical output lines 2110 are selected by the analog switch circuit2100, and output signals are output to an output line 2200. The analogswitch circuit 2100 can be sequentially selected by the shift registercircuit 1810 and the buffer circuit 1910.

In the circuit diagrams shown in the above embodiment, any of integratorcircuits shown in FIGS. 28A, 28B, and 28C may be connected to the wiring137 (OUT). The circuit enables an S/N ratio of a reading signal to beincreased, which makes it possible to sense weaker light, that is, toincrease the sensitivity of the imaging device.

FIG. 28A illustrates an integrator circuit using an operationalamplifier circuit (also referred to as an op-amp). An inverting inputterminal of the operational amplifier circuit is connected to the wiring137 via a resistor R. A non-inverting input terminal of the operationalamplifier circuit is grounded. An output terminal of the operationalamplifier circuit is connected to the inverting input terminal of theoperational amplifier circuit via a capacitor C.

FIG. 28B illustrates an integrator circuit using an operationalamplifier circuit having a structure different from that in FIG. 28A. Aninverting input terminal of the operational amplifier circuit isconnected to the wiring 137 (OUT) via the resistor R and a capacitor C1.A non-inverting input terminal of the operational amplifier circuit isgrounded. An output terminal of the operational amplifier circuit isconnected to the inverting input terminal of the operational amplifiercircuit via a capacitor C2.

FIG. 28C illustrates an integrator circuit using an operationalamplifier circuit having a structure different from those in FIGS. 28Aand 28B. A non-inverting input terminal of the operational amplifiercircuit is connected to the wiring 137 (OUT) via the resistor R. Anoutput terminal of the operational amplifier circuit is connected to aninverting input terminal of the operational amplifier circuit. Theresistor R and the capacitor C constitute a CR integrator circuit. Theoperational amplifier circuit constitutes a unity gain buffer.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Embodiment 5

In this embodiment, a structure example of a transistor that can be usedas the transistor described in the above embodiments will be describedwith reference to FIGS. 29A1, 29A2, 29B1, and 29B2, FIGS. 30A1, 30A2,30A3, 30B1, and 30B2, and FIGS. 31A to 31C.

<Bottom-Gate Transistor>

A transistor 410 shown in FIG. 29A1 as an example is achannel-protective transistor that is a type of bottom-gate transistor.The transistor 410 includes an insulating layer 209 that can function asa channel protective layer over a channel formation region in thesemiconductor layer 242. The insulating layer 209 can be formed using amaterial and a method that are similar to those of the insulating layer117. Part of an electrode 244 and part of an electrode 245 are formedover the insulating layer 209.

With the insulating layer 209 provided over the channel formationregion, the semiconductor layer 242 can be prevented from being exposedat the time of forming the electrode 244 and the electrode 245. Thus,the semiconductor layer 242 can be prevented from being reduced inthickness at the time of forming the electrode 244 and the electrode245. According to one embodiment of the present invention, a transistorwith favorable electrical characteristics can be provided.

A transistor 411 illustrated in FIG. 29A2 is different from thetransistor 410 in that an electrode 213 that can function as a back gateelectrode is provided over the insulating layer 118. The electrode 213can be formed using a material and a method that are similar to those ofthe electrode 243.

In general, the back gate electrode is formed using a conductive layerand positioned so that the channel formation region of the semiconductorlayer is positioned between the gate electrode and the back gateelectrode. Thus, the back gate electrode can function in a mannersimilar to that of the gate electrode. The potential of the back gateelectrode may be the same as that of the gate electrode or may be a GNDpotential or a predetermined potential. By changing the potential of theback gate electrode independently of the potential of the gateelectrode, the threshold voltage of the transistor can be changed.

The electrodes 243 and 213 can both function as gate electrodes. Thus,the insulating layers 117, 209, and 118 can all function as gateinsulating layers.

In the case where one of the electrode 243 and the electrode 213 issimply referred to as a “gate electrode”, the other can be referred toas a “back gate electrode”. For example, in the transistor 411, in thecase where the electrode 213 is referred to as a “gate electrode”, theelectrode 243 may be referred to as a “back gate electrode”. In the casewhere the electrode 213 is used as a “gate electrode”, the transistor411 can be considered as a kind of top-gate transistor. Furthermore, oneof the electrode 243 and the electrode 213 may be referred to as a“first gate electrode”, and the other may be referred to as a “secondgate electrode”.

By providing the electrode 243 and the electrode 213 with thesemiconductor layer 242 positioned therebetween and setting thepotentials of the electrode 243 and the electrode 213 to be the same, aregion of the semiconductor layer 242 through which carriers flow isenlarged in the film thickness direction; thus, the number oftransferred carriers is increased. As a result, the on-state current andthe field-effect mobility of the transistor 411 are increased.

Therefore, the transistor 411 has large on-state current for the areaoccupied thereby. That is, the area occupied by the transistor 411 canbe small for required on-state current. With one embodiment of thepresent invention, the area occupied by a transistor can be reduced.Therefore, with one embodiment of the present invention, a semiconductordevice having a high degree of integration can be provided.

Furthermore, the gate electrode and the back gate electrode are formedusing conductive layers and thus each have a function of preventing anelectric field generated outside the transistor from influencing thesemiconductor layer in which the channel is formed (in particular, ablocking function against static electricity).

Since the electrode 243 and the electrode 213 each have a function ofblocking an electric field generated outside, the charge of chargedparticles and the like generated on the insulating layer 109 side orabove the electrode 213 does not influence the channel formation regionin the semiconductor layer 242. Therefore, degradation in a stress test(e.g., a negative gate bias temperature (−GBT) stress test in whichnegative charge is applied to a gate) can be reduced, and changes in therising voltages of on-state current at different drain voltages can bereduced. Note that this effect can be obtained when the electrodes 243and 213 have the same potential or different potentials.

The BT stress test is one kind of accelerated test and can evaluate, ina short time, a change caused by long-term use (i.e., a change overtime) in characteristics of transistors. In particular, the amount ofchange in threshold voltage of the transistor between before and afterthe BT stress test is an important indicator when examining thereliability of the transistor. If the amount of change in the thresholdvoltage between before and after the BT stress test is small, thetransistor has higher reliability.

By providing the electrode 243 and the electrode 213 and setting thepotentials of the electrode 243 and the electrode 213 to be the same,the change in threshold voltage is reduced. Accordingly, variation inelectrical characteristics among a plurality of transistors is alsoreduced.

The transistor including the back gate electrode has a smaller change inthreshold voltage by a positive GBT stress test in which positiveelectric charge is applied to a gate than a transistor including no backgate electrode.

In the case where light is incident on the back gate electrode side,when the back gate electrode is formed using a light-blocking conductivefilm, light can be prevented from entering the semiconductor layer fromthe back gate electrode side. Therefore, photodegradation of thesemiconductor layer can be prevented and deterioration in electricalcharacteristics of the transistor, such as a shift of the thresholdvoltage, can be prevented.

With one embodiment of the present invention, a transistor with highreliability can be provided. Moreover, a semiconductor device with highreliability can be provided.

A transistor 420 shown in FIG. 29B1 as an example is achannel-protective transistor that is a type of bottom-gate transistor.The transistor 420 has substantially the same structure as thetransistor 410 but is different from the transistor 410 in that theinsulating layer 209 covers the semiconductor layer 242. Furthermore,the semiconductor layer 242 is electrically connected to the electrode244 in the opening which is formed by selectively removing part of theinsulating layer 209 overlapping the semiconductor layer 242.Furthermore, the semiconductor layer 242 is electrically connected tothe electrode 245 in the opening which is formed by selectively removingpart of the insulating layer 209 overlapping the semiconductor layer242. A region of the insulating layer 209 which overlaps the channelformation region can function as a channel protective layer.

A transistor 421 illustrated in FIG. 29B2 is different from thetransistor 420 in that the electrode 213 that can function as a backgate electrode is provided over the insulating layer 118.

With the insulating layer 209, the semiconductor layer 242 can beprevented from being exposed at the time of forming the electrode 244and the electrode 245. Thus, the semiconductor layer 242 can beprevented from being reduced in thickness at the time of forming theelectrode 244 and the electrode 245.

The distance between the electrode 244 and the electrode 243 and thedistance between the electrode 245 and the electrode 243 in thetransistors 420 and 421 are longer than those in the transistors 410 and411. Thus, the parasitic capacitance generated between the electrode 244and the electrode 243 can be reduced. The parasitic capacitancegenerated between the electrode 245 and the electrode 243 can also bereduced. According to one embodiment of the present invention, atransistor with favorable electrical characteristics can be provided.

<Top-Gate Transistor>

A transistor 430 shown in FIG. 30A1 as an example is a type of top-gatetransistor. The transistor 430 includes the semiconductor layer 242 overthe insulating layer 109; the electrode 244 in contact with part of thesemiconductor layer 242 and the electrode 245 in contact with part ofthe semiconductor layer 242, over the semiconductor layer 242 and theinsulating layer 109; the insulating layer 117 over the semiconductorlayer 242, the electrode 244, and the electrode 245; and the electrode243 over the insulating layer 117.

Since, in the transistor 430, the electrode 243 overlaps with neitherthe electrode 244 nor the electrode 245, the parasitic capacitancegenerated between the electrode 243 and the electrode 244 and theparasitic capacitance generated between the electrode 243 and theelectrode 245 can be reduced. After the formation of the electrode 243,an impurity element 255 is introduced into the semiconductor layer 242using the electrode 243 as a mask, so that an impurity region can beformed in the semiconductor layer 242 in a self-aligned manner (see FIG.30A3). According to one embodiment of the present invention, atransistor with favorable electrical characteristics can be provided.

The introduction of the impurity element 255 can be performed with anion implantation apparatus, an ion doping apparatus, or a plasmatreatment apparatus.

As the impurity element 255, for example, at least one kind of elementof Group 13 elements and Group 15 elements can be used. In the casewhere an oxide semiconductor is used for the semiconductor layer 242, itis possible to use at least one kind of element of a rare gas, hydrogen,and nitrogen as the impurity element 255.

A transistor 431 illustrated in FIG. 30A2 is different from thetransistor 430 in that the electrode 213 and an insulating layer 217 areprovided. The transistor 431 includes the electrode 213 formed over theinsulating layer 109 and the insulating layer 217 formed over theelectrode 213. As described above, the electrode 213 can function as aback gate electrode. Thus, the insulating layer 217 can function as agate insulating layer. The insulating layer 217 can be formed using amaterial and a method that are similar to those of the insulating layer205.

The transistor 431 as well as the transistor 411 has large on-statecurrent for the area occupied thereby. That is, the area occupied by thetransistor 431 can be small for required on-state current. With oneembodiment of the present invention, the area occupied by a transistorcan be reduced. Therefore, with one embodiment of the present invention,a semiconductor device having a high degree of integration can beprovided.

A transistor 440 shown in FIG. 30B1 as an example is a type of top-gatetransistor. The transistor 440 is different from the transistor 430 inthat the semiconductor layer 242 is formed after the formation of theelectrode 244 and the electrode 245. A transistor 441 illustrated inFIG. 30B2 is different from the transistor 440 in that the electrode 213and the insulating layer 217 are provided. Thus, in the transistors 440and 441, part of the semiconductor layer 242 is formed over theelectrode 244 and another part of the semiconductor layer 242 is formedover the electrode 245.

The transistor 441 as well as the transistor 411 has large on-statecurrent for the area occupied thereby. That is, the area occupied by thetransistor 441 can be small for required on-state current. With oneembodiment of the present invention, the area occupied by a transistorcan be reduced. Therefore, with one embodiment of the present invention,a semiconductor device having a high degree of integration can beprovided.

Also in the transistors 440 and 441, after the formation of theelectrode 243, the impurity element 255 is introduced into thesemiconductor layer 242 using the electrode 243 as a mask, so that animpurity region can be formed in the semiconductor layer 242 in aself-aligned manner. According to one embodiment of the presentinvention, a transistor with favorable electrical characteristics can beprovided. Furthermore, according to one embodiment of the presentinvention, a semiconductor device having a high degree of integrationcan be provided.

<S-Channel Transistor>

A transistor 450 illustrated in FIGS. 31A to 31C has a structure inwhich a top surface and side surface of the semiconductor layer 242 bare covered with the semiconductor layer 242 a. FIG. 31A is the top viewof the transistor 450. FIG. 31B is a cross-sectional view (in thechannel length direction) taken along dashed-dotted line X1-X2 in FIG.31A. FIG. 31C is a cross-sectional view (in the channel width direction)taken along dashed-dotted line Y1-Y2 in FIG. 31A.

With the semiconductor layer 242 a provided on the projection of theinsulating layer 109, the side surface of the semiconductor layer 242 bcan also be covered with the electrode 243. Thus, the transistor 450 hasa structure in which the semiconductor layer 242 b can be electricallysurrounded by electric field of the electrode 243. In this way, thestructure of a transistor in which the semiconductor layer in which thechannel is formed is electrically surrounded by the electric field ofthe conductive film is called a surrounded channel (s-channel)structure. A transistor having an s-channel structure is referred to asan s-channel transistor.

In the transistor with an s-channel structure, a channel is formed inthe whole (bulk) of the semiconductor layer 242 b in some cases. In thes-channel structure, the drain current of the transistor is increased,so that a larger amount of on-state current can be obtained.Furthermore, the entire channel formation region of the semiconductorlayer 242 b can be depleted by the electric field of the electrode 243.Accordingly, off-state current of the transistor with an s-channelstructure can be further reduced.

When the projecting portion of the insulating layer 109 is increased inheight, and the channel width is shortened, the effects of the s-channelstructure to increase the on-state current and reduce the off-statecurrent can be enhanced. Part of the semiconductor layer 242 a exposedin the formation of the semiconductor layer 242 b may be removed. Inthis case, the side surfaces of the semiconductor layer 242 a and thesemiconductor layer 242 b may be aligned to each other.

As in a transistor 451 illustrated in FIGS. 32A to 32C, the electrode213 may be provided below the semiconductor layer 242 with an insulatinglayer interposed therebetween. FIG. 32A is a top view of the transistor451. FIG. 32B is a cross-sectional view taken along the dashed-dottedline X1-X2 in FIG. 32A. FIG. 32C is a cross-sectional view taken alongthe dashed-dotted line Y1-Y2 in FIG. 32A.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Embodiment 6

In this embodiment, examples of an electronic device including animaging device of one embodiment of the present invention will bedescribed.

Examples of an electronic device including the imaging device of oneembodiment of the present invention are as follows: display devices suchas televisions and monitors, lighting devices, desktop personalcomputers and laptop personal computers, word processors, imagereproduction devices which reproduce still images and moving imagesstored in recording media such as digital versatile discs (DVDs),portable CD players, radios, tape recorders, headphone stereos, stereos,navigation systems, table clocks, wall clocks, cordless phone handsets,transceivers, mobile phones, car phones, portable game consoles, tabletterminals, large game machines such as pinball machines, calculators,portable information terminals, electronic notebooks, e-book readers,electronic translators, audio input devices, video cameras, digitalstill cameras, electric shavers, high-frequency heating appliances suchas microwave ovens, electric rice cookers, electric washing machines,electric vacuum cleaners, water heaters, electric fans, hair dryers,air-conditioning systems such as air conditioners, humidifiers, anddehumidifiers, dishwashers, dish dryers, clothes dryers, futon dryers,electric refrigerators, electric freezers, electricrefrigerator-freezers, freezers for preserving DNA, flashlights,electric power tools such as chain saws, smoke detectors, medicalequipment such as dialyzers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), and vending machines.Further, industrial equipment such as guide lights, traffic lights, beltconveyors, elevators, escalators, industrial robots, power storagesystems, and power storage devices for leveling the amount of powersupply and smart grid can be given. In addition, moving objects and thelike driven by fuel engines and electric motors using power fromnon-aqueous secondary batteries are also included in the category ofelectronic appliances. Examples of the moving objects are electricvehicles (EV), hybrid electric vehicles (HEV) which include both aninternal-combustion engine and a motor, plug-in hybrid electric vehicles(PHEV), tracked vehicles in which caterpillar tracks are substituted forwheels of these vehicles, motorized bicycles including motor-assistedbicycles, motorcycles, electric wheelchairs, golf carts, boats, ships,submarines, helicopters, aircrafts, rockets, artificial satellites,space probes, planetary probes, and spacecrafts.

FIG. 33A shows a video camera, which includes a first housing 941, asecond housing 942, a display portion 943, operation keys 944, a lens945, a joint 946, and the like. The operation keys 944 and the lens 945are provided for the first housing 941, and the display portion 943 isprovided for the second housing 942. The first housing 941 and thesecond housing 942 are connected to each other with the joint 946, andthe angle between the first housing 941 and the second housing 942 canbe changed with the joint 946. Images displayed on the display portion943 may be switched in accordance with the angle at the joint 946between the first housing 941 and the second housing 942. The imagingdevice of one embodiment of the present invention can be provided in afocus position of the lens 945.

FIG. 33B shows a mobile phone, which includes a display portion 952, amicrophone 957, a speaker 954, a camera 959, an input/output terminal956, an operation button 955, and the like in a housing 951. The imagingdevice of one embodiment of the present invention can be used for thecamera 959.

FIG. 33C shows a digital camera, which includes a housing 921, a shutterbutton 922, a microphone 923, a light-emitting portion 927, a lens 925,and the like. The imaging device of one embodiment of the presentinvention can be provided in a focus position of the lens 925.

FIG. 33D illustrates a portable game console, which includes a housing901, a housing 902, a display portion 903, a display portion 904, amicrophone 905, a speaker 906, an operation key 907, a stylus 908, acamera 909, and the like. Although the portable game console in FIG. 33Dhas the two display portions 903 and 904, the number of display portionsincluded in a portable game console is not limited to this. The imagingdevice of one embodiment of the present invention can be used for thecamera 909.

FIG. 33E shows a wrist-watch-type information terminal, which includes ahousing 931, a display portion 932, a wristband 933, a camera 939, andthe like. The display portion 932 may be a touch panel. The imagingdevice of one embodiment of the present invention can be used for thecamera 939.

FIG. 33F shows a portable data terminal, which includes a first housing911, a display portion 912, a camera 919, and the like. A touch panelfunction of the display portion 912 enables input and output ofinformation. The imaging device of one embodiment of the presentinvention can be used for the camera 919.

Needless to say, the examples are not limited to the above-describedelectronic devices as long as the imaging device of one embodiment ofthe present invention is included.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

This application is based on Japanese Patent Application serial no.2014-143256 filed with Japan Patent Office on Jul. 11, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An image sensor comprising: a pixel portion including a first photodiode, a second photodiode, a first transistor electrically connected to one terminal of the first photodiode and one terminal of the second photodiode, and a second transistor electrically connected to the one terminal of the first photodiode, wherein the other terminal of the first photodiode is electrically connected to a first conductive layer, wherein one of a source and a drain of the first transistor is electrically connected to a second conductive layer without passing through any transistor, wherein one of a source and a drain of the second transistor is electrically connected to a second conductive layer without passing through any transistor, wherein the other of the source and the drain of the second transistor is electrically connected to a third conductive layer without passing through any transistor, wherein the other of the source and the drain of the second transistor is electrically connected to a third conductive layer without passing through any transistor, wherein a photoelectric conversion region of each of the first photodiode and the second photodiode is closer to a light incident side than the first conductive layer, the second conductive layer, and the third conductive layer, wherein the first conductive layer and a gate electrode of the second transistor overlap with each other, wherein the second conductive layer and the gate electrode of the second transistor do not overlap with each other, wherein the third conductive layer does not overlap with the semiconductor layer of the first photodiode, wherein the third conductive layer does not overlap with the gate electrode of the second transistor, wherein the first conductive layer and the fourth conductive layer are positioned at different heights, wherein the second conductive layer and the third conductive layer are positioned at different heights, wherein the third conductive layer and the first conductive layer are positioned at different heights, wherein a width of the first conductive layer is larger than a width of the fourth conductive layer, and wherein a width of the second conductive layer is larger than a width of the third conductive layer.
 2. The image sensor according to claim 1, further comprising a driving circuit comprising a third transistor, wherein a channel formation region of the first transistor and a channel formation region of the third transistor are positioned at different heights.
 3. An image sensor comprising: a pixel portion including a first photodiode, a second photodiode, a first transistor electrically connected to one terminal of the first photodiode and one terminal of the second photodiode, and a second transistor electrically connected to the one terminal of the first photodiode, wherein the other terminal of the first photodiode is electrically connected to a first conductive layer, wherein one of a source and a drain of the first transistor is electrically connected to a second conductive layer without passing through any transistor, wherein one of a source and a drain of the second transistor is electrically connected to the second conductive layer without passing through any transistor, wherein the other of the source and the drain of the second transistor is electrically connected to a third conductive layer without passing through any transistor, wherein the other of the source and the drain of the first transistor is electrically connected to a fourth conductive layer, wherein a photoelectric conversion region of each of the first photodiode and the second photodiode is closer to a light incident side than the first conductive layer, the second conductive layer, and the third conductive layer, wherein the first conductive layer and a gate electrode of the second transistor overlap with each other, wherein the second conductive layer and the gate electrode of the second transistor do not overlap with each other, wherein the third conductive layer does not overlap with the semiconductor layer of the first photodiode, wherein the third conductive layer does not overlap with the gate electrode of the second transistor, wherein the first conductive layer and the fourth conductive layer are positioned at different heights, wherein the second conductive layer and the third conductive layer are positioned at different heights, wherein the third conductive layer and the first conductive layer are positioned at different heights, wherein the fourth conductive layer is farther from the light incident side than the third conductive layer, wherein a width of the first conductive layer is larger than a width of the fourth conductive layer, and wherein a width of the second conductive layer is larger than a width of the third conductive layer.
 4. The image sensor according to claim 3, further comprising a driving circuit comprising a third transistor, wherein a channel formation region of the first transistor and a channel formation region of the third transistor are positioned at different heights.
 5. An image sensor comprising: a pixel portion including a first photodiode, a second photodiode, a first transistor electrically connected to one terminal of the first photodiode and one terminal of the second photodiode, and a second transistor electrically connected to the one terminal of the first photodiode, wherein the other terminal of the first photodiode is electrically connected to a first conductive layer, wherein one of a source and a drain of the first transistor is electrically connected to a second conductive layer without passing through any transistor, wherein one of a source and a drain of the second transistor is electrically connected to the second conductive layer without passing through any transistor, wherein the other of the source and the drain of the first transistor is electrically connected to a fourth conductive layer, wherein the other of the source and the drain of the second transistor is electrically connected to a third conductive layer without passing through any transistor, wherein a photoelectric conversion region of each of the first photodiode and the second photodiode is closer to a light incident side than the first conductive layer, the second conductive layer, and the third conductive layer, wherein the first conductive layer and a gate electrode of the second transistor overlap with each other, wherein the second conductive layer and the gate electrode of the second transistor do not overlap with each other, wherein the third conductive layer does not overlap with the photoelectric conversion region of the first photodiode, wherein the third conductive layer does not overlap with the gate electrode of the second transistor, wherein a fifth conductive layer and the gate of the second transistor overlap with each other, wherein the first conductive layer and the fourth conductive layer are positioned at different heights, wherein the second conductive layer and the third conductive layer are positioned at different heights, wherein the third conductive layer and the first conductive layer are positioned at different heights, wherein the third conductive layer and the first conductive layer are positioned at different heights, wherein the fifth conductive layer and the first conductive layer are positioned at different heights, wherein the fifth conductive layer and the second conductive layer are positioned at different heights, wherein the fifth conductive layer and the third conductive layer are positioned at different heights, wherein the fifth conductive layer and the fourth conductive layer are positioned at different heights, wherein a width of the first conductive layer is larger than a width of the fourth conductive layer, and wherein a width of the second conductive layer is larger than a width of the third conductive layer.
 6. The image sensor according to claim 5, further comprising a driving circuit comprising a third transistor, wherein a channel formation region of the first transistor and a channel formation region of the third transistor are positioned at different heights. 